My 68 z is 07b rear code BV0316G1 E engine is V0628mo on pad. CAST E228. It looks like rear diffs. were stretched out more
Looking at your rear code of being a 4:10, that is certainly a less common ratio option to order, IMO, even back in '68. Your date spread is no worst than Joe's (X66 714) cars that too have steep ratios. I would venture to say yours (and Joe's) are within a very acceptable range.
As for engine dates vs. cowl dates, I find it interesting how the small block Z's are so close compared to my BB dates (and other BB dates I have seen in the past). I guess it is partially dependent on when the model options 'catches on' to the buying public.