I'd adjust the valves and since you have a vacuum gauge use it to set the timing, just set to hightest vac reading. If this doesnt work Id go through the tune up proceedures in the old shop manual. Also I have a mini HEI, instructions suggested plug gap .060-.080, plug type extended tip type plugs.

Best of luck
Jim

It's easier to get forgiveness than permission!
1946 1/2-Ton Chevy
1953 Chevy 3/4-ton Factory Stakebed