You'll have a lot less hassle if you find a Q-jet off a 400. Jetting on a 425 is probably off enough to cause some poor running qualities. It might not, but tuning it could be frustrating. 400's are common enough in the wrecking yards, most have 4-bbls. If you want to go the cheapest route, grab the intake too. At stock 400 revs, there will be little difference in power compared to an aluminum intake, and it may even run better, especially when the engine is cold in the morning.