You shouldn't have any problems regarding if or how it will fit. 1970 Chevy's used the TH350 so floor and other clearances are fine.
Just my perspective on things: why not keep it a standard but use a 4 speed? I find rowing through the gears a lot more fun and it gives your left leg something to do while you're in the car.
You must already have the clutch pedal and all the linkage if the car is now a 3 speed.
