Driveshaft length

That is the last thing you should do. The car should be complete, all interior and at you specified ride height. I say this because there are different rears, tire size, ride height, weight, trans length etc. So I couldn't say what length you would need and also the factory shaft may be abled to be used, again depending on what trans, yoke size of trans, rear yoke size etc. Not trying to pop the bubble, but you really only want to do this 1 time and it be right.