Can't.Unless you are a machinist.You will need to mill out the frame bolsters to literally nothing to get them into the metal parts.Your best bet IS the new trucks, and on the top pivot plate, isolate the shorting tabs, and run an extra pair of wires back to wherever.That parts is easy.
If you want to mount the plungers with new crossmembers, you will need to buy a new 2-truck Shay with all those bits, tear them both down, and try to re-assemble one from the resulting good and broken parts.Getting those parts out of Bachmann is probably not going to happen.Want to split the frame rails?First, like before, buy a new 2-truck and tear it all the way down.Then, remove trucks, firebox/ashpan, truck pivots, cylinders, end steps, boiler mount screws, wires to trucks, end beam under plates, rear cross support, then cut, pry or break every floor support bracket on both sides of the frames off, any piping that fits into the frames needs to come off, then start prying apart to see what's stuck.Two weeks later when you give up, you can go back and just add 2 more wires to each end, and in the process get the bonus of not having those 4-pin contact blocks to give you issues.
Seiously, you are making this a LOT harder than it is.
And, the last thing you want is those plungers.
We rip them out and hard-wire for reliability anyway.
Springs overheat and collapse, the plungers will cut a groove in the circuit board eventually, you'll end up with all sorts of headaches.
Side terminals are track, end terminals are motors.
Find where the pivot plate circuit board is jumpered (PC track), cut it, add two wires per truck, even if you have 6 thumbs per hand it will take you an hour to do.
I've done it, piece of cake.
I have also had a Shay down to 2 frame rails (guy dropped his and bent his frame, wanted new rails, when he found out how much work it was he passed.....so I had to put it together again) and it is seriously beyond the scope of most folks abilities.
TOC
This is a subject that i do not know near as much about as i would like too. Your idea of getting an MTS ready chassis is the way i would go but i reiterate i don't know to much about this and have limited abilities in this direction.
Rgds Ian
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