i would either paint the whole sbc with weldable primer, or wait till they are installed and shoot them with some spray can under the car.
if you spray where they are going to be welding, unless with something that says you can weld over like weld-thru primer , they are going to have to grind it down where they are welding anyway to get metal to metal contact.
i cant see weld thru spray can primer being more then like, 6 bucks a can, and you just need enough coats to cover it, it doesn't need to look pretty, being under the car.