I have used 68 #17 heads and 70 #11 heads with good results. The 17's on a 400 and the 11's are on my sons 64 Bonneville 389. They are not bad heads for a street car with a mild hydraulic cam. Add screw in studs and big valves to the 47's and you got essentially a #48 head (11's = 12's etc.) The best thing about using small valve heads for cores is that installing large valves allows the valve to be set where the factory intended. (Large valve heads after many years and unknown valve jobs tend to have the valves a little to recessed for optimum performance without having new seats installed which drives up cost. Large valve head cores are usually more expensive too!) The ports are the same. So yeah keep your 47's.
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