351's have a larger bolt for holding on the heads when compared to a 289/302. You need something to take up that space when you slap a 351 head on a 289/302.

 

yeah, see my previous post LOL

think of it like a worn out woman, you can throw a hotdog down a hallway ya just need a board tied to your butt so ya dont fall in....... does that make any more sense?

really, you dont even need washers under the heads of the bolt but i wouldnt build on without them. keep in mind the heads are aligned to the block by the dowel pins, all the head bolts are doing is keeping the squeeze.

 

but still thats alot of force for those dowell pins