For the level you want and sticking with a B wedge, there's not too much available except Keith Black. I would suggest you go with and RB and then use the World iron block. The cost will be about 1/3 more the cost of properly prepping the 40yr old stuff, but it is MUCH stronger than the end result. If you reach well beyond 600, factory blocks are on the edge of strong enough, even in the B design. The secret is great parts, and perfect machining. The lower end parts will not survive long term in the build you want. You will need a very good crank (I do not consider 440source as great. good, but not great), a main girdle, and the bores need to be thick. As far as sonic testing, I own my own good unit, and test regularly. The minimum thickness I would look for for your level is .200 on the major thrust, no less than .150 on minor thrust and on the pin axis. Unless you get lucky, your block will not have that. Sleeving can help, but they have to do it right. I would estimate you will need to test 2-3 blocks before you find one that will be good enough by those numbers without sleeving any holes. And make sure your tester gives you at least 20 readings per hole, plus the minimum scanning thickness. This lets you see the trends in the core placement on that block. The best 400 block is the cold weather 400 last three digits of "230", and cast before 1972. Those are thicker. You can also run a 1/2 fill and an oil cooler too. That will also make a marginal block be usable. But again, you have to make sure the upper segments of the cylinder are thick enough. A 1/2 filled block, with the thickness above and a girdle, with a good forged crank, should live at 600n/a, and another 300-400 under spray. Block prep alone will approach the $4000US level if you have it all done with normal machining (fit caps and girdle, align bore/hone square deck, bore, plate hone, lifter bore work, clearancing for stroke), oil system mods, and lifter bore correction and bushing. Or you can call and get a KB b wedge block, fully machined and in aluminum for a bit more.
