[Dingfodgy]

Unless there is something heinously wrong with the '72 frame- why not just swap sheet metal? Leave the '72 frame as is and make it all purty if you like and box it where you want/need. Once that is done pull the bed, cab and front clip off your '68 and mount it to the newly freshened up frame. Slap in your motor and trans and head on out to the track for some serious radial destruction!
I haven't done this yet (it's on my "to-do" list) but it seems the coil frames have a little kick-out where the springs mount that the leaf trucks don't have. I've seen guys get some fabbed bag/coil mounts and weld them in, which is what I'll probably do. You'll also need to come up with a way to mount a panhard bar, being as the leafers didn't have one. There are kits out there, but like most things...they ain't free!