Welcome to CHR!

I’d be concerned about a .060 over bore in a 400 long before I’d be concerned with the 2 versus 4 bolt mains. As Jerry stated, they are a good candidate for the splayed 4 bolt conversion and I certainly agree with him that it would hold the lower end together however; but bring a bunch of money to your machine shop. Was the block sonic tested after the boring? The .060 over on the Siamese blocks is really pushing it as their design (two cylinders actually touching) is not as efficient in the cooling department and they are prone to “hot spots” where the cylinders touch. Removing .060 doesn’t leave a lot of cylinder wall.

That said – you didn’t tell us what you have in mind for the engine – when you say “one run” does this mean you’re going to drag race this engine? Why 550 HP? What is this engine going to go in?

Before cam selection it would help to know; rear end ratio, tire size, transmission (manual or automatic) and stall (if auto) would be good to know as well.

Give us a bit more info and there’s a lot of help on this site!

Again, welcome to CHR,
Glenn