I just spoke with the guy who did my 4 speed conversion. He said it could also be that he overtightened the 2 screws that connect the Hurst mechanism to the aluminum piece on the transmission.

He likes to really tighten those 2 bolts because of past experience with a 4 speed on his old 70 Road Runner. The bolts were too loose, and the holes had to be helicoiled after too much spirited shifting.