I have a 66 Duetto with transmission internal parts from a 74 GTV, and a 86 Spider. They are interchangeable over decades, with just a few small differences. The reverse switch is in the bell housing in the early ones, and on the tx on the later ones.
The shift rods go through the bell housing on the early ones - to actuate the reverse switch.
The reverse lock-out changed around 1970 from a push-down-on-theshift-lever.
But almost any one can be used with only small considerations.