82 GMC Engine Swap

I just bought a 82 Sierra 1500 with a straght 6 that's bad. It's an automatic with a 350 trans? How can I tell? Can a small block work with the straight 6 tranny if it's not? Will this swap be difficult?

Need to exchange Heater Core also. What's the difficulty level in this?

I'm a 5yr brain cancer winner on a once a month budget needing to do most of the work myself. Any suggestions?