454 Chevys are harder and harder to find these days. A 455 Pontiac in it would be neat, you should be able to find a suitable candidate in a boneyard, or pull my favorite trick and buy a whole car, running or not. That way, you can use the core support, radiator, shroud, fan clutch, fan, pulleys, belts, motor, transmission, driveshaft, differential, wiring, switches, relays, lights, yada, yada. Sell off the remainder of the sheet metal to recoup some of your investment.

As Shawn mentioned, you could keep it in the GM family by popping in a Caddy. I think '70 to '74 will net you a 472 or 500, '75 to '76 no 472's were made, only 500's. But I'll tell you one thing, even a 472 will make 500 ft/lbs of torque stock and that's nothing to turn your nose up at.

http://cad500parts.com/
http://www.500cid.com/