I'd trust a 400 or a 4L80 in a street driven truck, wouldn't run any automatic on a trail or in a tow rig though. I just don't trust them. Seen them leave people high and dry WAY too many times, and it's never convenient.
But back to the original topic, definitely check out the front end/steering, heater controls, and tranny. Also, see if it's a 10 bolt rear end, or a 14bsf. I've seen 14bsf's in 1500 and 2500 trucks, so you never know. I definitely wouldn't want a 10-bolt though, if you plan on hauling stuff much.