Anyone know how to tell from casting numbers if my crankshaft is a 400 or a 351m crank.. I bought a spare engine last week and the heads are 400's. pretty sure its a 400 engine but i wanna make sure the crank is def a 400 before i take it to the machine shop. thanks

 

Can you measure the crank throw? 2 inches is the magic number.

Scroll down about 2/3 of the page here for the casting number etc. info:

M-Block 351M/400 Parts Reference

 

Heads the same for both.

 

400 crank have a ridge on the first conterweight. The stroke is 3.5" for the 351m and 4" for the 400. Stroke is the center line of crank to center line of connecting rod or piston travel.