I'd go the Olds route. Like tyoneal said you could get a used 455 easily and not do much to it to make power. The prices for parts are outrageous but you may be able to find deals. Nothing like the deals on the SBC, though. Dick Miller is a racer and has a wealth of knowledge on the sites.
www.dickmillerracing.com
And around these parts a 200r4 is a mere pullapart and $50 away. That's right, $50.