Vacuum on 1968 GTS 340 4 spd.

OK, good you tested that. I'd not use the RockAuto PCV; I would bet it will be wide open at the cruise flow opening with the lower idle your cam. I've used a Fram 191 (made for factory cammed-up GM's, that transitions at the 11-12 in range) but those are for larger valve cover grommets than the stock SBM grommet.