Just measure it carefully and do it, no biggy. The front cab mounts are the same, just a bolt and a pad, and if the rears are different swap out or use an S10 mount pad between rear floor crossmember and top of frame. Run a strip of masking tape across the back of the cab even with the top of the box and cut bed mount blocks to hold bed in same position on 53 frame. I wouldn't pay for more than one days time for this job. Here is a shot of one just backwards of your plans; a 53 bed and front sheet metal on a 55 first series (note one piece curved windshield).

http://s32.photobucket.com/user/coilover/media/Picture018.jpg.html

Evan