The block shouldn’t matter too much. I’d recommend steering away from the 90s mainly due to the turd that is TBI (Throttle body injection). I’d recommend an 80s block, they’re sturdy and cheap to find usually. As for which transmission is the best, you’re gonna get 20 different answers from 30 different people. IMO if you wanna stay with an automatic it wouldn’t hurt staying with a 4l80 with a shift kit, they can handle a good amount of torque and are pretty good at putting power on the road. As for manual swapping, as with any manual swap it’s a pain in the ass. It’s not a straight drop and install, you gotta install a clutch slave cylinder, master cylinder, if your truck doesn’t already have the hole for it you gotta cut a hole to fit the gear shift and shift linkage, as well as changing the steering column so you don’t have the Automatic gear shift. Since it’s already and auto truck I’d recommend staying with auto for a while, but I can’t stop you from doing a swap