It looks like your heads have an accessory bolt hole. I couldn't make out any other casting marks in the pictures. I think it might be an early 350 with a even earlier intake manifold. If you pop a valve cover off you can get the casting number and be sure of the heads. Your thermostat housing spacer looks like some part off a marine application where they use water cooled exhaust manifolds or something. If the engine is a marine application you might have a stout bottom end to work with. Get some casting numbers off the block and heads and you'll solve the mystery.
SimS
