I would invest in a gasket set and open that baby up, if only for inspection and cleaning. It might be rare/valuable enough to freshen up: hone, rings, bearings, valve job, etc.
How many miles on it?
Double-check your timing of #1 at TDC of compression stroke.

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If you can't fix it with a hammer and screwdriver, you need a bigger hammer.
1965 Chevy C10