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those distributors get dry and rusty around the upper shaft and advance won't work smooth and even advancing/retarding-----and q jets get build up of crud around jets/metering rods and idle...

Change any rubber sections of fuel lines, filters, maybe even put on a new pump because modern fuels probably have destroyed those hoses and diaphrams

I have also used some of the Holley spread bore carbs--usually tho on a Chrysler back in the 70s

guaranteed vacume leak

Remember that if it has a quadrjet that you'll need to change the manifold if you go square bore holley--------