[olddog]

The 390 has the same stroke as the 427.

The 391 truck has a larger OD on the snout (where the damper goes) and will need turned down. The huge advantage is that these are steel cranks -not cast iron. Now I cannot remember for certain if all 391FT are steel, but most are, if not all.

Unless you have some rules preventing you, why not stroke it out? Cost about the same for a whole rotating assembly either way.