If its been rebuilt then its anyone's guess. sounds like you have new pistons, rings, bearings, cam, etc. you may not even have the original heads or intake. Since all that's changed it being from a '79 vette is essentially meaningless. I'd check to see what cam you have, what head castings you have, what pistons you have, do all the research and investigating you can now and it'll save you a LOT of work and headaches later.
Since you're stuck with what seems to be a weaker engine I would keep an eye out for a high stall and some gears. You'd be surprised how fast you can go with little power given enough stall and gear.