Measuring DS length w/o yoke

Car is a 65 Signet. I have the 360/727 installed and the stock 7 1/4 rear.
I'm thinking about running the small rear for a little while to break in the engine and need a drive shaft. I don't have a 727 yoke and the measurement between the end of the 727 output shaft to center of the u joint straps is ~50.5".
This is with full weight on back tires. How much length do I need to subtract for the yoke? Thanks