74" engines have a stroke of 331/32", and 80" shovelhead engines have a 4¼" stroke,
but the cylinder length for both engines is the same. The 74" piston is taller than
the 80" piston; piston deck heights of the two pistons are designed for the same
piston to head clearance in both engines. While it is possible to install an 80" top
end on a 74" engine, this will result in very low compression and poor performance.
A 74" top end will bolt to an 80" crankcase, but, the pistons will contact the cylinder
head due to the combination of the taller 74" pistons and the longer stroke of the
80" flywheels. Therefore, it is important to correctly identify the stroke of the
engine before ordering a top end kit.