The last new bytes in a GCR stream are unused, that's what I mean, these can used. 325 doesn't evenly divide, remember sectors are GCR bytes and not an even number of bytes.
As for G64, having them byte aligned is a bug. Many protections take advantage of tracks not being perfectly byte aligned. It breaks V-Max and Datasoft protections, Pirate Slayer, etc. I'm sure there is others that mess around with stuff in the tail gap, and check tail gap size. If you can think it up, someone did it.
Tons of Talent cracks don't work because of the disk id thing, there a loader that was written on the directory track. I fixed a few of them, Mason did too. Lots of old old cracks don't work either because of it.