2 bolt main will do just fine for this build and even more. Everyone makes a huge issue of 2 and 4 bolt main. Here is what's way more important than 2 or 4 bolt main. BALANCING !!! You can even rattle a 4 bolt main loose if it's not balanced right, but 2 bolts hold up to alot more abuse than most realize. I've spun 350' making power to 7000 rpm and even out to 7500 rpm without problems w/ 2 bolt main and cast crank. Now, that's not a big block but still, point is Balance a motor right, lighten the rotating assembly where you can and it'll hold up.
Personally, I prefer 2 bolt block anyway. In the event I think I'm building something that needs 4 bolt, then I'd much rather go with Splayed mains and angle bolts. You can't do that on a 4 bolt block because it's already been drilled straight down and taken meat away where the angled bolts need to be machined. I also think a main cap girdle has more to offer than 4 bolt mains. For those who aren't aware of what a girdle is... It looks similar to a ladder and lays across all of your main caps and is bolted down and torqued with the mains. This keeps all the mains standing up straight and your center mains aren't able to walk or rock, not to mention more strength because now your main caps are thicker.
At any rate, balance it right, even though big blocks have a heavy mass to rotate, they are bigger mains and can handle it even with a cast crank.