I found out on the 67-72 Chevy Trucks forums that an '88 to '91 spindle with 1.25" rotors will work with an earlier IFS ('67 to '70). It will need a lower a-arm from a similar year ('67 to '70)3/4 ton because of the larger ball joints. The studs may be 14mm, so the rears may have to be drilled to match.

Are the '67 to '70 suspensions "bolt-on" like the '73 to '87s so that they can be removed from the donor vehicle?

Thanks for the info.