Why not rebuild the old 750???Was it a double pumper??
Pretty much what ever you buy will need to be taylored to your motor anyway,so I would just(unless its shot)rebuild the existing carb using propperly matched power valves,then jet it and play with ecelerator pump cams.
If you do buy one I'd say just a 750 double pumper.I think the factory Z28s came with a 780 DP.