Just adding to this old post for anyone who searches for this topic in the future.

If you are going with that approach, I can suggest starting with these Hillman axle bolts #882944.
After cutting the hex head off, there is about 1.312" length left.
The ones I bought from a local hardware store were 0.4965-0.4970 dia. So I sanded the block end to my satisfaction.
A little bevel on the end helps get them started.

I am using these 1 1/2" long bellhousing dowel pins from Summit in my Mega block with no modifications to pins or block.They had a good press fit and tapped in easily and smoothly.After checking crank/trans run out only had .003" so no offset dowels needed, just extra long. Summits filled the bill.
https://www.summitracing.com/parts/sum-192002 Bill