Does anyone know if I can use a 1967 rear coil spring in a 1966 frame assembly? Is the frame configured differently, or is the 67 spring beefier?

You can use pretty much use any pigtail design spring from any car ever built, just depends on how much rate and how high you want it to sit. Most GM intermediates run about the same installed height (some CC style sit higher) and they all run about 115lbs of rate.

They are not interchangeable. The 64-66 frame has a full diameter coil pocket. 64-66 rear springs are pigtailed only on the bottom. The 67+ spring is pigtailed top and bottom.

Rene
66_Malibu

Yeah, I use either spring style on both chassis'. As long as the pigtail is on the axle end of the seat the frame will accomodate either style.