If you can find an '85 FSM on-line (I'm sure it's out there somewhere), there are detailed frame diagrams in the back, with dimensions, showing where all the mounts are.
You could put a reg cab on the extra cab frame, but the rear body mount will have to be moved forward, and you'll have to find a longbed to fill the gap at the back of the cab. Or you could slice about 8" out of the middle of the frame. But this will cause all sorts of other problems, like your driveshaft will be too long, as will the exhaust.
Once you have an Xtra cab, you will never want a reg cab. I will never own one again, that's for sure.