Transmission swap help

This weekend I just brought home a 1968 Camaro.
My question is this...

The car has a 2-speed power-glide transmission in the car. I believe that it was originally a Muncie as there is a clutch pedal hung in the car, but I am not sure.

But one of my first orders of business is going to be swapping out that transmission for something with a few more gears, and looking towards a manual shift again (since I like throwing gears). And was wondering what would be a good choice.

I guess my main concern would be to keep something that would require as little work as possible since I am on a slightly limited budget at the moment, but would like to have it going down the road by next spring.

Do I need to find a transmission with the same overall length? Would changing the driveshaft be that big of a deal?
Would a 6-speed from a newer F-body or Vette work?

Any help please.
This is my first classic camaro, and looking to make sure I get it right.