you must be kidding me..........i picked up a 67 nova with a 250 in it a few years ago........the nova would always run rough....i took off the old carb rebuilt it.....it didnt help...ran rough....went through the whole car...coil..plugs..wires...on and on and on.....i had a few single barrel carbs lying around and starting messing around with them....5 total .....no change in rough idle running.....the intake manifold where the gasket goes between manifold and carb is flat as is the manifold top..(
no notches on carb intake)...so this went on for years...yes years...i found a 250 6 cyl. out of a 67 camaro with all the goodies....power steering...carb....etc...so this carb was the same specd. for the 250.....well guess what....the car now idles nice......if i would have rejeted the first carb i had i would have found the problem earlier.....i must admit i did have one small crack in vacuum line from intake manifold to valve cover and that was 1/2 the problem....i hope that helps....that thing tormented me for a few years............