1972 Longhorn

[Tonner]

I recently picked up this '72 Longhorn that once belonged to my Grandparents. I have been working on a '72 C/30 flatbed. I got i running and driving, but the body is too far gone to be worth resurrecting. I'm going to steal the 350 out of it and throw it in this truck to get it running. I'll drive it for a while as I figure out how much I want to modernize it. It came with the original 350 in pieces in the bed. I'm going to have a machine shop check it out as it has been rebuilt before and see if it is worth working with. My question is how much horsepower can I realistically get to the ground if I want to keep the stock ride and length? I keep watching shows like this and thinking I could give it some "get up and go" without getting too crazy.
https://www.youtube.com/watch?v=f7mvgf6_p1g
I want it to be fairly reliable and not a total gas hog. Should I just keep a stock 350 in it, or build it a bit?