If you have a SM420 4 sp transmission, (used by GM forever) I believe the cheapest and easiest swap to better gears is a different rearend and a latter model rear coverplate and yoke for the tranny. I know the SM 420 gearbox is the same from the late 40's to at least the late 60's. Anyone after 54 will have the rear coverplate and driveline yoke for an open driveline. All you have to do is swap them on torque tube model. (This is not true for the 3sp)
Find your replacement rearend based on width and gearing. You will have to weld up spring perches. Note, due to torque tube hinge action, the spring mounting pins, and thereby the axle, will sit too far forward if you center the axle over the pins. Some say you can flip the springs end to end to cure this. I just offset the perch mounting holes. Also note the stock axle location in the wheel well, they sit forward anyway. The axle does move back under load. Watch your pinion angle when welding perches.
Finally,have a driveline fabbed up at a driveline shop. It will have to have a slip spline in it.
I believe this is the easiest and cheapest way to go. A T5 certainly would be nice, but they are more expensive and you have to deal with some modifications, (front shaft and retainer, matching clutch parts, hole in floor and,depending on the year T5, the speedo gear.) And you still would need to do all the same things to switch out the rearend and driveline.
As with all things automotive, it jsut comes down to what you want and what you can spend. With my '50, a mostly stock weekend work driver, it made sense to stay with the old 4 sp.
Steve O
