Well, there's good news and bad here. I've turned the barrels down to 13/16's and started to drill the monoblock out to the same dimension. But, my drill press has stalled. I had no problem going 5/8's and then 11/16's with HSS bits but no luck with a 13/16's cobalt. I guess I missed 'Drilling 101'! The suggestions seem to be drill your pilot the size of the bit's web and then go with the final....unless you are going to ream. Still I guess you can't exceed the horsepower of your drill press.....mine just stalls.
Lesson 2: Plan your build! I bought the AyA 12 ga. because it had the features I wanted, AND the price was right! Plus I wanted a large bore. After, I found a set of barrels in .45 would cost more than some used doubles. So, went to .30 cal. Problem is 30 bbls on 12 ga action are not going to be as pretty. The rib is going to be quite wide. It was suggested a 20 ga would have been better! It's still fun, though.
I'll have to take my block to my local machine shop and let them bore, drill....whatever, to 'hog through.