My '68 Fastback 4 speed swap

Since you have the motor out, you might want to check your bell housing alignment. I did this on my factory 4-speed Challenger and found it out of tolerance alot. I had to have the factory dowel pins pulled out of the 440 so I could install 0.014" offset pins to correct the crank run out.

If it's out alot, it'll be hard to shift and add extra wear to the input shaft bearing.

Attach a dial indicator to the crank flange and set the needle to follow the inside of the trans bearing retainer hole. Have a helper rotate the engine over slowly with a breaker bar and socket on the damper nut while watching the dial indicator swing thru it's range and note the position of the min and max points. Write down the min and max numbers and subtract. That's your run out.