Excellent, You've inspired me!
Been looking to do same. Several months back, I snagged a Power Torque Industries cast iron version of this OD unit with thought of also placing in front of my SM420. I have the add'l task of re-swapping back in my 3.90 ring/pinion.

How did you go about modding the driveshaft?

1960 Chevrolet Apache 10 Short Fleet black
1995 Chevrolet Silverado 1500 Z71 RCSS black
1963 Oldsmobile Starfire Coupe black/black
1965 Oldsmobile Starfire Convertible teal
1965 Oldsmobile Starfire Convertible white
1965 Oldsmobile Dynamic 88 Convertible sand
1965 Oldsmobile Dynamic 88 Convertible red