Help on my 1964 4 speed swap

Just to reiterate, you only cut the sheet metal, not any part of the trans crossmember.

Way back (yeah, I'm old) I put a 4-speed in a 65 Valiant that originally was a three on the tree. Brewer, et al, didn't exist then. I just used a later A body hump, cut out of an original floor shift car. Probably a Duster, but can't remember for sure. Anyway, with a little massaging it fit and cleared the bench seat.

By the way, you should crawl under a factory four speed early A body sometime and check how crudely the floor pan was cut for shifter clearance.