Sure looks like an SM-420 4-speed to my untrained eye. To be certain, check the part number on the drivers side (591665). Attached are some pictures of the SM-420 from my '49 3100 you can compare.
Regarding your questions:
1. SM-420 I'm pretty sure
2. Depends on your plans for the truck. On one hand, if it works, you could say don't mess with it. But I can tell you when I took mine open it was full of sludge. Likely never having been open before, and who knows what different oils were in it over the years. It also leaked like a sieve, which I wanted to at least slow down. Here are some pictures I took while working on mine -
SM-420 Transmission In a more direct answer to your question, besides just checking the oil level, it might be a good idea to slide the universal ball cover back and check to be sure the universal joint bolts are tight.
3. I would strongly recommend you get (or make) guide pins. I tried for a while to get my transmission lined up without them, but finally gave up. To make them, just get longer 3/4" bolts, about 3-4" I think, cut the heads off and screw them into a couple of the mounting holes. This will help you get everything lined up.
