I had to deal with something very similar to this on my 70 Chevy-built 350cui engine. I changed the intake over to a 0-50/Performer-400 Edelbrock intake.. this(& along with the different carb, Crane-valve-train, special-ground cam, pistons, ported & polished heads, HD-50 oil-pump, etc), changed the vacuum ratios on the engine which caused mistiming on the distributor's vacuum advance. it took a sharp fellow I knew some 'shiny' math to finally get it all right. I finally had him build me a correct vacuum advance diaphragm unit that worked perfect.
..and after having to re-jet the new carb.. lol
Michael