you picked a great guy to ask. but allow me to butt in.
my truck is a 4x4 now. it was a 2x five years ago. i had to cut 8 inches out of my frame to get it under my 56, which is in my photo album. the way to do it is obviously you need a flat surface. take lots of measurements from key areas, front to back and across diagonally to keep your frame square. then you want to make your marks, use a template i spoze would give you the same distances. pick a part of the frame that is flat and straight. you want to make your cuts at a diagonal, not straight up and down as i did, although i don't think it matters much. then weld it together with some good rod, then some filler rod. grind it flat, then you want a solid steel plate welded over both sides of the frame, inside and out, that covers the whole weld. this is called a fishplate for some reason. i would say put a straight edge as long as you can on the top of your frame before you weld it together. get someone who can weld good if you cant. it was easy. you want to cut enough out so that your wheels are in the centers of your wheel wells front and back. peace out bro