i talk to a fellow who was around y blocks all his life and he tells me of someone years ago who used a 292 block and had the mains honed to 312 size and used a 312 crank.he knows about cutting down a 312 crank for a 292 main.anyone ever hear of this.

Yes, I have done this a few times, but the price of 312 main bearings makes it more economical to grind the crank to 292 size.  Also, to put a 312 crank in a 292 block either requires machining the rear main seal area to 312 size or machining the crank seal area to 292 size.  Also the slinger at the back of the 312 crank has to be cut down to fit into the groove in the block.  Unless you have the capability to do the machine work yourself, it isn't really a good idea.

John - "The Hoosier Hurricane"

i didnt think anyone would have done this.you never know.

I did this for a couple of reasons.  In building engines for my race car I ran out of 312 blocks that would bore .030, so I used 292 blocks bored .080.  I didn't grind the crank to 292 size because I had found cracks in 312 cranks from my race car  in the radii of the main journals.  I had broken a couple of cranks.  I also wanted to maintain the overlap between the rod and main journals.  I noticed that most crank grinders eliminated most of the radius because they wouldn't dress a radius on their grinding wheel.  So I used 312 cranks which were still standard in the 292 blocks.  I now know a local  shop that will preserve the radius for me when he grinds my cranks.  My next build (if there is one) will probably have 292 size mains on the 312 crank. 

John - "The Hoosier Hurricane"