E-e-easy, now. Take a deep breath and remember why you got the thing in the first place.
Start with something known to be good...is the floor level? Got 4 identical jackstands? Flashlight? Tape measure?
Let the suspension "dangle" and measure from the floor to frame at lots of points, especially where crossmembers are. And don't forget those diagonal measurements. We're not worried about 1/16" here or 1/8" there...
If the frame is reasonably straight, double-check those cab & bed mounting points: rubber parts, bracketry, etc. A couple shims won't hurt if that's what it takes.
If the truck looks good up on stands, maybe the suspension is the culprit. Check the springs, perches, shackles, bolts, etc.
Am I saying anything you haven't already thought of? Sometimes just seeing/hearing it makes it look/sound a little easier. After all these years, could you really give up? Just think how cool it'll be to drive...even if it takes a little longer.
Best,
cm
