#13's with 72 cc chambers on a +.030 400 - if everthing else is "factory nominal" (pistons .020 "down the hole", .042 head gaskets, 6 cc's in the piston valve reliefs) you'll be at 10.065:1 compression. There are people running that much compression successfully on pump gas (you need a pretty radical cam to get away with it), but personally - I wouldn't try it.
Leave everything else the same, but instead run pistons with 12 cc's of dish (D-shaped, not round) puts it at 9.509:1 --- much safer --- and the D-shaped dishes mean you don't sacrifice your quench pad area - this is a good thing.
If you have 6x-4's (98 cc's nominal) you'd have to cut them down to 85 cc's, AND zero deck the block, AND run thinner .035" gaskets to get to 9.417:1 compression on a +.030 400. I'm not sure how much they can be safely milled, and then too you start getting into issues where you have to do additional machining to make the intake manifold fit right, and get shorter pushrods to make the valve geometry right... it adds up. However, those thin gaskets make for a very tight quench area which is heap big goodness.
HOWEVER - build that 400 into a stroker (461), run those 6x-4's untouched at 98 cc's, run flat top pistons, zero deck the block, and run .035 gaskets - that puts you at 9.430:1 with very good quench --- and what's not to like about 461 cubic inches?
From what you describe, chances are by the time you spend the money to get those #13's into useable shape you're going to have spent enough coin to get a good ready to run set of 6x-4's...