I agree. Most casting numbers are generic and used on many different bore blocks. The C5AE-6015-A is one of them as you see. I have seen some of them go to 4.13" std 406-428 bore but 4.23" would not be likely. And having the cross bolt bosses is a nice option to crossbolt w/factory or aftermarket caps. Some of the C5AE-A blocks were also solid lifter blocks used in the 390 PI engines. Some 352's had that casting number too. Dave Shoe's drill bit test will tell you aprox what you have for cyl wall cores. But as Shoprat said a sonic map is best for exact results. Nice find on that virgin. Not many of those around anymore.