You have found a truely rightous block there my friend. A CNC prepped bowtie block is a factory race prepped block. It comes from the facorty in two bores, 4.250" and 4.495". It is safe to bore these blocks to 4.630". So you are very safe on the bore. You have the making of a monster motor. But don't be fooled, the rortating a$$embly< crank, rods and pistons> to complete the build can cost as much as a completely built 454. A 502 is a 4.500" bore with a stock 4 inch stroke crank. Go to a 4.250 stroke and you got yourself a 540 cuin. If it were me I would go ahead and bore the block to 4.560". Drop in a 4.250 stroke crank and VIOLA!! You got a 555" firebreather!
This engine would be rock solid in mild form and potential would be unlimited. Leave in the stock stroke crank and bore to 4.560" and you got a 509". Also a bulit proof combo!
I will gladly help you make a beast out of this thing.
By the way, that block would cost you around $2000 from chevy's hiperformance center. If you don't want it, I'll take it.
One more thing. A tall deck bowtie block is what is needed to build a large inch stroker. The production tall blocks were only used in heavy trucks. They are ok as long as you don't get too crazy, but the bore size is limited for a street engine. The BOWTIE blocks are cast specifically for race use and the block is stronger in all areas. To use a tall deck you have to run longer rods. And a specific intake manifold. The heads are the same. Those heads are not going to feed nearly enough air into this engine. You will need larger intake ports and open chamber style heads, for many reasons. Like I said, If you decide to go with this get with me and I will get specific for you on what to look for.