Mark;
I don't know a thing about the 239 Pontiac engine, but I do know a little about engines, re-building same, and engine swaps.
It's an old engine, and if parts are available, should be rather easy to re-build (it sounds like you've done some research if you have the costs).
Reliable? sure, if you treat it well, and drive it within it's design limits - which brings me to your question of 'is it fast enough for a cruiser?'. Define 'fast' - cruise all day at 65mph? If so, that might not be the best idea, and the brakes and suspension will be a little past their limits.
Last, 'pull it out and put something else in'. You will have some major surgery to perform to install it, and you'll have to match a transmission to the new engine.
If I remember, that year Pontiac is an open rear, and fairly strong, so the rear end should hold up; but a higher ratio might be desirable.
If you car is standard shift*, and you have a limited budget along with limited knowledge, then re-build the engine and use that as learning experience.
*I mentioned the transmission because if it is automatic I think that would make it an early Hydromatic which may have it's own problems, and will probably cost more than the engine to repair.
Just my .02 - but you did ask...........
