New main caps or old

At that level aftermarket caps aren't really needed. If you planning on stepping it up and going over 700hp on a B block, then start looking at main caps.

Here's another thing to chew on. If you do go over the 700 mark, start thinking about an aftermarket block. I spent a good chunk of change on alum caps, machine work on a 400 block. It would have been a bit more to get a better piece to start with.