The SM420 4 speed is as tough as nails. They came in everything from 6 cylinder 1/2 ton to GMC diesel powered 2 tons. The "SM" stands for Synchro Mesh. Which means the gears are synchronized, no double clutching required. The only gears that aren't synchronized are 1st and reverse. With a 7:1 first gear, I don't think you'll be needing it much on the street.
The 4x4 guys love them because of their strenth and ultra low first gear.
I wouldn't hesitate to put a 350 in front of one. I wouldn't get crazy and throw nitrous and a blower at it. A 300 hp engine would do fine paired with it.
The transmission has a wide spread on the gears, so you're no going to be able to ram and jam like a Muncie, but they are practically bullet proof otherwise.
http://www.fourwheeler.com/how-to/tr...ons-and-cases/
http://www.advanceadapters.com/tech-vault/gm-sm420/