Awesome buy! Looks like a pretty clean car, that's far more important than what engine is in there. I'd rather have a clean 318 car than a rustbucket with a BB.
You could always just get a stroker kit for it too, 318's go out to 390 with a 4" stroke. That and some heads with 2.02/1.60 valves would make plenty of power! And it would retain the original block.