Well they would have to run a way lower compression ratio for one. No engine with a 10:1 will last long with any kind of pressure added to it. I beleive the sc version had something like 8:1 compression ratio and a tighter oil clearance as well.
As it goes for rebuilding these engines, it's perfectly possible. Don't listen to that quote from that article which was posted above. There is alot of false statements in it. First off northstar heads are only throwaways if they are cracked. Sure why spend the money on parts and labor to replace valve guides? (Which they make replacement ones btw). Any good technician, will throwaway a head with worn guides, if they are looking for a good reliable rebuild. Your better off buying heads with good guides, than messing with the worn tired ones on yours. Whether you have a 305 chevy or a 5.0 mercedes engine. Chances are if the guides in your heads are worn and cracked, the rest of your heads are probably in the same condition. Better off rebuilding a cleaner, lower mileage example. As for the bottom end, true there isn't undersize or oversize "main bearings" available, but you can find rod bearings to all type of specs (the ones that go bad the most). Chances are if you spin a mainbearing the engine just became a good boat anchor anyways. Now true these engine's arn't commonly rebuilt, ofcourse! Parts are expensive as hell, and the price of rebuilding one is almost as much as just buying a long block and putting it in. Thats the only reason they are commonly just replaced. Now if your a tech or a do it yourselfer and you have the knowledge to rebuild one you can do it and save yourself a decent amount of money.