Be very careful pushing the limit on compression. There are too many variables that go into the mix to be able to say with confidence "you can always run X with Y octane". Climate, altitude, cam shaft (biggie), tune up (fuel mixture), ignition timing, the shape of the combustion chambers, piston deck height (just to name a few) all play a part. Generally, it's not worth trying to push the limit anyway. Consider this: the 461 in my car (the Beast) is making probably somewhere in the neighborhood of 520 - 530 HP right now (it dynoed at 495 before I changed to the aluminum heads). Pushing the compression by a full 5 points (going from 9.5:1 to 10.0:1 for example) would only buy me about 8 hp on this motor. On a street engine, taking that much risk for less than 10 hp gain isn't a good idea, in my opinion.
Also, those 4x heads will yield different compression ratios on different engines. You've got to know some details to calculate what the real CR is, namely bore, stroke, compressed head gasket thickness, head gasket bore size, piston deck clearance, piston dish/valve pocket volume, and actual combustion chamber volume.
For instance, 4x heads came in two chamber sizes (nominal - actual sizes can vary quite a bit - this is why you have to measure them) - 98 cc's or 114 cc's
On a 400 with "nominal" measurements (.020 deck clearance, standard bore/stroke, 6cc's in the valve pockets, .042 compressed gasket, 4.16 gasket bore) 98 cc 4x's will yield a 7.961:1 CR. 114 cc's would be 7.129:1
72 cc 670's would be at 9.935:1
If the engine is bored +0.030 those numbers change to 8.063, 7.218, and 10.065 with the same heads
On a "standard/nominal" 455 with those three chamber sizes you'd be looking at 8.925, 7.978, and 11.170.
Be aware that the "compression ratio" topic is one of those that tends to arouse very high emotion and much debate amongst gear-heads. Everyone has an opinion that they're passionate about, some of it grounded in experience/fact, some of it purely emotional. In the end it comes down to how much risk you're willing to take and how much effort/money you're willing to put into making it work. There are people (although a very precious few) who are getting away with 10:1 "or so" with iron heads on pump gas with the "exactly right" parts on a "meticulously maintained" combination. There are also others with engines that have detonated themselves to death at less than 9.5:1 with the "wrong" combination of parts and sloppy maintenance.