My '73 had the two factory low backs installed when I bought it. Wanting something a little more comfortable for my delicate behind, I perused the innards of a little junk yard and grabbed some surprisingly clean subaru bucket seats for a mere 15 dollars each (sliding, adjustable back (not that it's really of any use)) and with some flat steel bars and a drill, I was able to modify the subaru seats to fit the factory mounts. It all depends on the bolt pattern, really, if you're going to stick some seats like that in there, but it most certainly can be accomplished.
'Bout a 3 hour weekend project, and the seats are easily interchangeable when needed.
Just a thought,
P.S. The passenger seat is still able to fold forward for removal (and storage compartment access), despite the addition of a headrest.