So many things to answer, so little time…
First, length from end beam to end beam on Shays can be an issue on tighter curves with overhang on body mounts.
Check to see where the centerline of the coupler falls between the rails, then check the cars…
Plus, truck-mounted couplers lead INTO the curve, body mounts lead OUT of the curve, exacerbating the situation…
AMS on Shays, yes, done them, no problem, as long as you have the mahogany to make a new lower beam and 8 brass wood screws, and 4 old Big-Hauler long body screws.
Take the bottom plate out of the end beam, cut your new wood beam to match in length the plastic factory ones, lay the plate on the beam to mark the holes, drill clearance holes and recesses, install late, glue new beam to plate, insert screws, tighten and align.
Then set the engine on the track with a gauge or AMS car for height, fit coupler, cut out necessary recess for box, pre-drill screw holes and mount.
Piece of cake.