Sounds like you've got a mismatch of generations/applications goin' on there :
The back seat seat back frames(#6)
up to the end of 1972 & from 1973 on were different as well as all convertible frames were from hardtops,so since you've already had them recovered what I'd do is line up the stoppers & latch the crossrod then devise some extensions from steel flatstock(welded to or bolted through the frame you have) to be able install the pivot bolts(#14) .
I've seen this done before but just can't find any mention of that specific fix right off the bat .
Other stuff I looked at toward that end :