[FFR428]

Some of the 391 blocks are known to have really thick cyl walls. Not all but some. They also have the triple webs at the crank saddle. The best way to check is sonc the block for thickness. So be careful what you buy. If you find a nice block try to get it local so you can have it checked. Be sure the seller is willing to do so too. this will give you a idea if he's FOS or not! G.