Ah, but here you're falling into one of the common misconceptions about cosmology. The universe isn't expanding *into* anything -- if it were, the universe would have a centre
First of all, I never suggested that the universe actually *was* expanding into anything... I only offered the notion that even if it *were*, the fact that it has only been doing so for a finite amount of time still makes the universe finite, even if the space it were expanding into were infinite.
Second of all, if it were expanding into an infinite space, there is no reason to conjecture that this space would be limited to three dimensions, so no "center" to our expanding universe can necessarily be found - any more than you could find the center of a soccer ball on the surface of the soccer ball. A center definitely exists, but it's not going to be found on the restricted topology of the soccer ball, and the center of the universe exists, but you can't find it at a point in space... it would therefore be most correct to say that the center of the universe is actually the instant in time of the big bang.