Transmission identification

[C101971]

I've got a 1971 gmc c15 with a 350 mated to a 4 speed. I've got a general idea of which tranny it is by the style of the shifter and the location of it is very close to the fire wall. I can't tell for sure without cutting out the floor or dropping the tranny, which I would like to avoid doing. I'm pretty sure it's either a Muncie sn420 or a sn465.

Reason I need to know is I want a Hurst shifter and I want to order the right part, also if I have to cut the floor I would like to avoid making any un necessary cuts.

There is also the chance that I have the complete wrong idea of what trans it is but there was only so many that are mounted that close to the fire wall and are a 4 speed.

Long question short, where is there an easy to see number to identify the transmission?