Well that's a blanket statement that is "not always true." LOTS of 327's were factory built for trucks and with non "camel hump" heads (AKA power pack) for less than "Corvette" performance. I don't remember what the lowest rated 327 was, 250? (EDIT wiki says 210 hp)
And 302s were no big deal, either. Hell we used to bore 283's and call them 301s!!!!!
But if you have a 283 (302) or a "truck" 327, you basically are up against the same costs as building a 318 in some ways..........you get to garbage can the heads and pistons.