You should have a gasket above and below the block. I don't know what Chevy had originally on your '58. Chevy used a block under the Rochesters on the Corvairs with gaskets above and below on both carbs. The later engines with four Rochesters used a total of eight gaskets and four blocks.
Even if you don't have a leak by not using two gaskets, you may crack the phenolic block. You will have a real leak then.
