Re: 351M Heads vs 351C heads
Mate if there the later "M" head the water jacket difference in in the exhaust ports that choke them down to a smaller X section than a windsor or cheb head!
Take them back and either compare inside the exh port around the valve guide to your original heads and if there is any diff ie;new heads smaller demand your old heads back!!
Ford mad and proud of it
71xyfmt, 96ef xr8, 82xe 's'pac 400k old workbus!