I recommend the 6x-4's. At 94 cc's chamber volume (please DO measure and verify yourself!!) on a +0.030 455 with flat top pistons and everything else "factory nominal", you'll be right at 9.3:1 compression ratio which is dang near perfect for pump gas.
If you're going to be replacing pistons "anyway", then consider having the block zero-decked and using pistons with D-shaped dishes and having a total of 10 cc's additional volume in them. That'll still have you at 9.3:1, but with the added benefit of excellent quench area characteristics (helps combustion efficiency and provides additional detonation "insurance".)
Another option: If you do zero-deck the block and use the 6x-8's instead and flat top pistons, you'd wind up just a hair shy of 9.1:1. You'd be leaving a little bit of power on the table (but not much, probably just a couple of HP) and still be getting the benfit of better quench.
Yet another option: Run the 6x-8's but have them milled .015 (again DO measure the volumes first so you know for sure where' you're starting from!). Your goal here is to get the chamber volumes "down" to 98 cc's (from factory nominal 101). Zero deck the block, run flat top pistons. This would put you at 9.3:1 and give you excellent quench properties.
Point is: You've got some very good options available to you depending on how much you want to spend on machine work and also the availability of a good machinist who knows this isn't "just like a chevy".