When I did mine I took 12" from under the cab, measured 24" 2-12" sections were marked off with a horizontal line down the center. That gave me a 4 section grid on the side of the frame. I cut the front upper section and the lower back out then overlapped the frame, this gave me a larger weld area, then added a 3/8" fish plate on the back side. Then you cut 8" off the back of the frame. Move the front bed mount back to the next bed mount and relocate the rear cab mounts. When I did my frame and finished the welding and grinding you could not see the splice. Mine was a 65 GMC with leaf springs so no problem with cross members. Do a search on 67-72 chevy truck page for all the details. I do not like butt welding frames, the overlapping method is much stronger. Just remember to measure twice or more and brace the hell out of your frame before cutting.
