[Sirrotica]

The late manifolds in an early chassis won't fit without cutting a large notch in the crossmember under the engine. I used RA III manifolds from a 70 Judge in a 67 chassis, and had to put a large notch in the frame to clear the outlets.

This is on the 67 GTO stock car in my signature pictures, because it was a race car sacrificing that frame section was insignificant. I would never consider it on a decent driver.

I've never owned a set of the 67 manifolds, so I have no direct experience in using early RA manifolds in a later chassis, sorry I can't answer that question.

Just an educated guess though, that Pontiac wouldn't have went to all the trouble to tool up redesigned manifolds for the later chassis, if the earlier ones would fit. They used the big car manifolds in the first gen Firebirds from existing patterns, and the second gen Firebirds will interchange with the later A body manifolds with out any changes, just common sense says those manifolds only fit the 64-67 A body cars, just because Pontiac only installed them in one year of chassis.