The 1967 axle used mono-leaf springs. 1968 and up used multi-leaf springs. The only difference is the spring perches, and those can be replaced as long as you have some welding skills.
I knew a guy that used a '67 rear in a '68 and he just used longer '68 U-bolts, and stacked washers between the perches. Not the best idea, but he got away with it.
The Chevy companion flange may be different too, but I remember there being a special U-joint that was half Chevy and half BOP.
