First, I had to fill in the hole in the firewall with some balsa and epoxy.
This is a good time to make sure none of those engine mount bolts are sticking out the back (very hard on fuel tanks: I learned the hard way!).
I had one sticking out on each boom since I had to narrow the mount a skoch to allow for muffler room.

I had to drill a 1/16" hole through the aluminum screw. I cut the heads off first. I then use two nuts and tighten them against each other to keep the bolt from spinning (also to hold the bolt in the vice). Center punching the screw and using a good quality bit are essential, plus exact perpendicular to the drill table (there's not a whole lot of slop allowed when drilling a 1/16" hole through a bolt that's only about an 1/8" diameter. The aluminum bolts are cheap though so no big deal if you break through the side. I drill half way through from both sides to reduce this risk and I use cutting oil to keep friction down (especially essential when drilling steel to prolong the life of those bits). Even if you break through a small amount, a little JB weld will fix her up (just make sure you don't block the vent hole up!).

I'm partial to the '3 line' method. Fuel, pressure, fill/drain.

The trusty H2O test. Plug up 2 holes, put the tank in water and blow and hope no bubbles appear. Of course, it's easier to do it now than when the engine goes out in flight!

NOTE: I'm using different fuel tanks that came with the ARF.

I'm using slant front tanks. The original tanks won't give you this problem. I finished up on the tanks but I didn't have a good feeling on the second tank. It felt a little tight near the fire wall. After pulling it back out, one of the lines got pushed against the brass tubing and was cut. With the 3 tube method, I had 2 of the tubes on top at about 10 and 2 o'clock and the third at 6 o'clock. The bottom tube at 6 o'clock came through at a good angle through the hole in the center of the fire wall. The other two tubes were too high so I took the tank out and bent the 10 & 2 o'clock tubes down about 45 degrees. This was just right and when the tank was put back in, the 3 tubes aligned with the center hole in the fire wall. There's enough room to see the tubes coming out and I made sure they weren't twisted with each other and then connected them up to the muffler and engine and let the fill/drain tubing hang down. I took the other tank out and did the same thing to it.

I used a foot long bit to drill through the fire wall and next former for the throttle servo control rod.

A little epoxy and the throttle servo will now be 3/8" higher.

Several bends were needed to get around the boxy muffler. A side benefit with this thin rod and the many bends was the throw of the servo arm and engine throttle didn't have to be exact. On full throttle, I get a slight bend of the wire but since the wire connects to the engine throttle at almost a 90 degree angle to the rest of the rod, it acts like a buffer.

When I tried to put the struts in the spring air retracts, the struts are a little too big in diameter, even when the set screw on the retract is completely removed. I ended up putting some masking tape 7/8" from the end to protect the gray paint while I hand sanded the exposed paint and some of the metal off.
I'm sure we'll have to do this again since it's unlikely that the length of the strut is exactly what is needed. (good time to get some quality bonding time with the other half by having her sand one strut while you do the other one!)

The 3 hinges were already glued into the rudders on my ARF. All I had to do was cut three slots in the vertical fin. The slots are already under the covering so not too much to do here. I made a flat side on one end of a popsicle stick and pushed the epoxy into the slots. Put a little oil on the center of the hinge pins and pushed the rudder's hinges into the slots. Nothing unusual on the control horns either.
The lower control rods are for the rudders. The outer servos in the booms are for the rudders. The clevises appear to be good quality so I just slipped on a piece of fuel tubing first and then slid the rod from the rear into the black pre-glued plastic tubes.

As you can see, I'm getting max rudder movement. Needed if one engine goes out. (later, we'll paint all those white parts black so they don't 'stick out' so much).

Next, I connected the horizontal stab to both booms.

The screws are 1" long. Each ends of the horizontal stab have pre-installed blind nuts. Test out the screws before connecting the booms. One end of the stab on my ARF had the blind nuts at a slight angle (not 90 degrees to the rib) so the screws had to go in at an angle. You don't want to force the screw in and cause the blind nut to come free: I've done that before and the only cure is to cut the head of the screw off so it can be fixed. I cut the covering inside of the rudder, where the stab is attached, about an 1/8" in. This allowed the stab to push that covering under it so it makes a nice clean appearance and also unlikely to tear in flight. The recesses on the outside of the rudder where the screws heads go are easily seen with the slight indentation of the covering. An x-acto knife was used to put an 'X' cut at this spot. Both control horns mounted. I use a manual modelers drill: going through balsa doesn't need anything more than 'hand' power.

Testing the elevator:

Elevator in neutral position.

Elevator in down position.

I cut a small notch in the former just behind the center throttle servo using
a razor saw. The 3 servo wires and 2 airlines (3 needed if you're not using Spring air retracts and also installing brakes) come out of the center wing directly over the servos. I was concerned about these wires eventually/possibly getting entangled with the servo arms. To prevent this, the wires and airlines go through that notch and connect with the 3 servos on the 'retract' side where there is lots of room. I used plastic ties to hold the servo wires together and routed them through the same former where there already exists a large hole.

Pretty straight forward on this. I used electric tape to tape the servo wire connectors to the wires coming out of the center wing, inserted the center wing dowels into the booms, aligned the wires and airlines in the slot we made, and put the 4 nylon screws in. Putting the booms on her nose made it easier to access the retract area to make sure all the wires and airlines are in that notch and nothing is being pinched. For now, the retract and brake lines are just looped out of the center hole of the center wing. We'll cut them later for connecting to the air valves. Leave several inches of air line sticking out of each boom so we can easily connect them up later. That 'rats nest' connecting to the receiver isn't too bad, considering how much 'stuff' we're operating!

Drop tanks.

Figure I might as well paint them up so they are thoroughly dry by the time we use them. Use 240 grit or finer to rough up the surface of the plastic to give the surface some 'tooth' for the paint to stick to. First coat is primer. Any of the spray paints at HD or Lowe's in a can will do, but we'll need to use a fuel proof clear coat for the last coat. Primer is important: it's virtually impossible to get a smooth coat of paint without primer.

Second coat: Black.
As with the primer, it's best to put on several light coats than try to cover it in one coat (also, less likely for the paint to attack the plastic).