Pinion angle and driveshaft length.

I just got off the phone with Denny from Denny's Drive Shafts and he says the exact same thing. Start with the car frame level and have both the trans and rear slope 2 degrees down. He said to ignore all the other stuff that is on the internet about matching angles up and down. He also said to set those angles before making the measurements. He has a worksheet on his website for all the measurements needed.