Is there any way to tell if a 351M has been rebuilt to a 400 without disassembling the motor? Are there external components that need to be added for a 400 rebuild? After doing some reading on the motors here, it would seem like common sense to rebuild it as a 400 if you're going to the trouble of rebuilding it.

 

Nope, externally the 351M and 400 motors are absolutely identical. The only way to tell for sure is to try and find the casting number on the crank. You can try to measure the stroke but it's imprecise at best.

 

Pull the oil pan and identify the crankshaft. Here's some info:

http://home.earthlink.net/~bubbaf250/parts/parts02.html

Scroll down to the section on crankshafts.

The only external component you might change for a 351M-to-400 conversion is the harmonic balancer.

 

Thanks for the link, it answered my question and more! Great site!