Actually, the '66 421 and 67-69 428 cranks are the same size (journals and stroke), but the 421 cranks are made from cast Arma Steel as opposed to cast Pearlitic Malleable Iron. The '65 and earlier 421 cranks are slightly different - although I don't know the details - but thay are also cast steel. The 455 and 428 cranks are physically interchangeable, with the exception of the 455's longer stroke. (Piston pin location is different between 455 and 428 pistons.)