I am swaping out the saginaw trans for a muncie m21. The transmission was built in 1967/1968, I have a 69 firebird. Should I buy the shifter for a 69 or 68.

I have an M22 with hurst competition shifter in my 69. One issue is the high left position of the shifter going into reverse . Went thru (3) different shift arms till I had an offset one that fit properly in the console.

You should use the '68 shifter. It's not the trans that's different, it's the console.