Sorry to hijack your thread.71swb4x4,If I remember correctly,the 69-71s had the rear frame like the trucks but the 72s had a closed in rear frame sorta like the front frame horns.I also think I remember reading that when you put a older 69-71 tub on a 72 frame that has the boxed in rear frame there is clearance issues.Hence the notches in the 72 tub for the boxed in frame...as circled in red below.
