The OD tailshaft mod (car overdrive) works on a later Saginaw 4 speed (the sideloader with external shift levers), but not on a SM420. If you already have a Chevy car overdrive, your options would be:
1.) Use the 3 speed chevy car overdrive
2.) Get a Saginaw 4 speed and adapt it to fit the car overdrive (just needs a hole drilled).
3.) Sell the car overdrive and buy a differant rearend gear and use the SM420.
4.) Sell both the overdrive and the SM420 and install a 5 speed tranny.
