Should it stay or should it go?

First year for the small block V-8 was 64. To keep the pushbutton shifter, you will need a 64 or 65 904 cable shifted trans, for the V-8.
The 65 trans will need the valve body changed to the 64 item. You can use the valve body from your 64 slant six trans. If the engine is 1968 or newer, you will need a adapter bushing in the end of the crank.
1966 and later trans is different, and can be converted to a pushbutton shifter by sending the PB unit to Imperial Services. It is not inexpensive.