Ford makes a head that fits the SS/CS spec of "Inline iron" called the N351. It is primarily for short track circle track racing. I did a lot of digging on this head and found it on the 1/8 mile record holding Mustang of Venice Perno.
It has a Ford part number, M-6049 N351, so it's not technically "aftermarket". Got mine from Speedway Motors. Require offset rockers and special headers.
links:
http://dragracecanada.com/ontario-ford-aficionado/
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