I haven't done a conversion myself, but I would list the options in this order;
1) Try to find a SWB 4x4 frame.
2) Find a LWB 4x4 frame and shorten it.
3) Use a 73 + frame and fab up new body mounts.
4) Jeep frame ????
SWB 4x4 frames are hard to find but there out there. There was one for sale on here this week but the front tips of the frame were cut off. Looked like a solid frame, and were it was cut I think wouldn't be to bad to fix. Cut the tips off of another frame and weld them on. ( Was cut just behind were the bumper mounted ) Just my 2 cents !!!
Try this link !
http://67-72chevytrucks.com/vboard/s...d.php?t=206233
Gary