Doug’s Headers on 64 A body 4 speed

That's the name of the game, you do what you have to do to make it all fit. Variance stack-up is always a consideration.
But using the short, late model biscuits does more than affect header clearances- even in cars with stock manifolds where exhaust clearances aren't as critical; will also alter (lessen) your clearance to the K frame and especially your tie rods/center link. Have seen situations where the tie rod ends' studs/nuts would hit the pan with the shorter mounts. Caveat Emptor.