A long shot, but did you change intake manifolds?
Back in the very early '70s I put a new intake manifold on my 340 Swinger and had a vacuum leak I couldn't find. The new intake had two sets of bolt holes for different brands of carbs. One of the bolt holes I wasn't using with my Holley was not completely covered by the carb gasket and when the bolt hole was factory drilled/threaded it partially broke through into an intake runner.
Can't hurt to check everything.
Good luck finding the problem.
