I asked because both decoders look the same, and both have pads for soldering.

I measured and the wheelbase on the athearn GP35 and P2K GP30 is the same. I actually test-fitted the athearn GP35 on the frame, lenghtwise it's a perfect, but the athearn shell is thicker, so the proto weight needs to be thinner, and some of the weights under the cab moved around.

As for putting the FT on it, the fuel tank is much different, and it would be a pain to make the mods for the FT shell to fit. I took apart one of them, but don't think my friend will be able to clone the chassis, and buying all the extra parts would probably cost more than the locos themselves.

How'd you know it was me?

I do have the ABB, but I also have 2 more As. I plan on using the AB set for the passenger train, and an ABA set for a freight train.

As for a P2K F7/GP9/30 shell, what's the point of putting a shell on another shell? 

I have a spare GP30 chassis with everything on it. (was to make the extra GP-30 shell of yours powered), but now I'm planning on putting it under my Athearn GP35 dummy, I just have to get a friend make the weight a little thinner (the athearn shel is thicker)

I did look for serial numbers on the board, the only difference is the one in the A-unit has a B at the end, and the B-units have a C at the end. I could run wires between the units but that would get annoying taking the consist apart.

Is there a difference between the decoder in the FT-A and FT-B?

I measured the current from the light outputs on the B units but there isn't any voltage, and was hoping to put my spare FT-A shell on one of the B-unit's chassis, and have LED lighting (already did it to the existing A-unit). Also the lesson learned from this will help with the FT-AB set I'm looking to put back-up lights on.

Is there something I can do with the circuit boards in the B-unit to lighting like the FT-A has?