Thanks GRJ! I was stalled until I got your reply as I was afraid if they were polarized I'd short out the AC Commander not knowing which leg was which.
In the end, the more I looked at the directions and the motor itself, I determined that my proposed connections from the center post of the motor ("field/common") to the new lugs (the green arrows in my post immediately above) were unnecessary. The screws I connected the new lugs to (one already had a wire holder on it that's visible on the right one in the photo I posted immediately above) are already grounded through the motor frame which is grounded through the armature and connected gear through to the wheels and the outside rails. The AC Commander instructions stated getting the AC Commander's "common" through direct frame connection "wasn't recommended." So I took the AC Commander common connection to the right side lug I added (the one where there was already a wire holder) where I connected the one side cap's common leg.
I'm not quite done yet and I only had enough time last night to try it out in conventional and verify it would go forwards and backwards and that the prime mover sounds were playing. I'll do a more detailed write up of what I did after I completely finished for anyone coming across this post in the future that wants detailed comments on how to do it for the first time.
But I went from this
To this:
In an afternoon.
The gap between the engine and heat sink of the AC Commander board (the big aluminum plates about the middle of the engine) isn't really unused space. As visible in the photo below, this is where the housing for the speaker goes. The plastic housing for the speaker takes up a lot of space but I felt it was worth it as I expect it provides better resonance for the sound. I positioned it directly below one of the smoke stacks to give the sound a good place to escape from. In a total piece of good fortune, the speaker housing was just the right diameter to wedge into the cab shell and stay there with nothing but friction. You can also see below where I soldered an antenna wire onto the outside hand rail that runs the length of the locomotive. I plan to add a second one to the other side before I finish, but the kit only included a single antenna wire.
The one big piece left concerns the coupler connections. In my one criticism of this kit, it seems to assume you're converting a PW celebration version of a locomotive with an AC motor where the coil couplers have two wire plastic connectors. But this 65 year old "original PW" engine has single wire couplers without connectors. I suppose I could try to solder onto the positive pin on the board connector but I'm not going to try that "kludge". I just need to find a source for the current style coil coupler connector with wires already attached and solder a connection to the existing wires.
