To give a bit of an idea what I was going for this is the complete unit:
View attachment 303582
It sounds like it is broadly similar to what you describe. The key difference is that instead of having the axle boxes move individually the side frames are a single piece and twist relative to each other. The left cross member has a single screw on which it tilts laterally, the right one is rigidly fixed to the car body. The side frames are held to the cross members by an M2 screw at each end. The central cross member is not fixed to the side frames, it just supports the back of the traction motor cases.
The holes in the cross members are all designed to line up with the existing screw holes in the bachmann tram car body, though some shortening of the plastic mounting posts will be needed to get the floor height right.
The reason why I went for a rigid side frame rather than separate moving axle boxes is it allows the use of a small flanged roller bearing in the axle boxes, as well as making the whole thing a lot stiffer for printing in PLA. For my printer at least I think the side frames need to be separate parts so they can print inside face down on the table. I could potentially make a version with the sideframes rigidly fixed to the cross members and separate axle boxes, though I'm not sure what orientation you would print it in to make sure everything was fully supported. I would think it would probably need to be ABS too, just because without the axle boxes in place (and presumably the primary suspension springs?) the frame on its own is pretty thin.
Hopefully this answers that as well.
I was going to wait until I've had a chance to print and test fit a prototype to make sure everything lines up properly before uploading to thingiverse but the sideframe on its own is attached if you want to see if you can make it fit your purposes.