make a jig. The last frame I shortened was an S-10 frame. I made a jig that picked up the front half of the frame section and the rear half. I drilled 4 locating holes in the frame. The jig had 4 additional holes located at the new location.
I took a photograph, drew dimensional lines and measured frame height, diagnals, lengths, etc. Then using a calculator, I had a list of measurements to check everything.
I ground out the factory welds cleaned up the frame sections, then trimmed, and re-assembled the frame. located the frame with the jig, verified the dimensions, tacked the frame, re-verified the dimensions, then welded it up.
If you are not shortening the frame where the factory front/rear connect (middle of a c-channel section) then you would need to use fish plates for every splice that is normal to the frame rail
