The four pics have a framing square laying next to black marks. That is the general area where I'm leaning towards cutting the frame.

Both frames, 55 and 91, are 6 inches tall. There is a 5 1/2 inch difference between the height of frame on 91 where the front cab mount should go after swap and the height of front cab mount on 55. The width of both frames are close, one frame might be able to slide inside the other. What I mean by height diff is measuring from floor up to bottom of frames.

If I'm looking at this right, the back cab mounts on 55 frame are going to stay......

All measurements are tentative. Just trying to get an idea of whats going on. Plan to measure more.

It looks like if I splice the frames straight up even, the front end is going to be low.