Are they the cast manifolds or headers? The headers on my 350 use I want to say a 3/8 pipe cap and their edelbrocks. My 351 ford I used either a 5/8 or 3/4 blowout plug from fastenal i think. The other chevy had a plug that was a bit smaller maybe a half inch? I took the ford manifold to fastenal and they found what I needed for me. You could just go to home depot and buy a half dozen different size pipe plugs and return what you don't use. Just if you use pipe plugs make sure you get black iron plugs rather than brass. The brass plugs expand more and will crack the manifold.
