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If it has a 4 bbl intake, can you tell if it is a spreadbore design? Fords spreadbore carbs were not the same pattern as the rochester spreadbores. Some of the earlier 429's had the 4300D spreadbores, while the 76 and up had the 4350 spreadbore carbs. I had a 76 460 van with a 4350 (so did my brother) and we spent many an hour working on those *#*$# 4350's. They had a square tube looking thing in the center of the rear barrels--weird looking. I believed they were rated less than 500 cfm's, even as a 4 bbl.
Regardless, you may need a 4300 series (70 351C's had a 4300C) difference in 4300C & 4300D was cfm rating, "D" was more. You may be able to adapt to a holly, or other, square bore with an adapter.