3951509 block:
The reason a two bolt it "stronger" on a 400 is because the bolts have a better clamping load since the webbing area is thicker. The 4 bolts crack more because the metal around the bolts is thinner compared to the twos.
I wouldnt worry about the bottom end thats the least of your problems. 400s crack between the bores on the deck more than anything else. Already have a junk block from that.
Sounds like a good 300-350hp combo, great street manners and reliability.