1970 chevy 2wd to 1970 chevy 3/4 4wd

[1970/327/c10]

Well its been a while since i last posted but here's the latest update. Got all the frame strip down ready to sandblast, bought a few parts like brake pads, brake shoes, calipers. Decided to tear into the rear end and looks like the brakes are just a little old but all the gear look good just needs new oil. And question of the day is if i can get my hands on a 400 small block would it be worth the trouble and time? I could also do a 383 stroker but what would be best for a k20 that i would use to tow things from time to time and fuel mileage? Also does anybody know how to decode these numbers they are under the passenger side?