Donf:

"The SM420 of the 48-54 variety is 2" longer then the original sliding gear crash box, so you need to figure out what to do with the extra 2" of torque tube."

Shortening the driveshaft and torque tube is not a problem. I just did that in my home shop when I used a '37 Chevy car rear axle in my '36 Chevy pickup in order to be able to use 3.55 gears from a '50-54 powerglide equipped Chevy car.

Also, it's been my experience that complete excellent used transmissions can often be found on ebay and craigslist for less than the cost of a single part from a professional vendor, often for MUCH less. For example, I recently bought a complete '36 Chevy synchromesh transmission in apparently excellent condition on ebay for $36 and I was the ONLY bidder. Those parts are scrap iron to street rod builders.

Ray