Backhalfing a car.......

I would strongly recommend purchasing mandrel bent, one piece rear frame rails.

When I did my son's '88 Mustang I reinforced the stock frame kick up, added new 2x3 from there back, and 2x3 forward protruding up into rear footwell and welded to front sub frame. Far more work than I expected should have bought the preformed frame rails.

The biggest decision, and argument we had was the tubs. He suggested for ease of fabrication is to build complete sheet metal ones, but with the rear window, like your Bobcat, I thought it would look too garbage canny. I cut them out, added flat metal to the side and a strip along the perimeter like a mini tub.

This retained the stock curved stamping and when you walk by and look in the rear window it's hard to tell how wide they really are. Turned out fantastic, worth the work and he agreed when I was done.