The 305 heads suck, don't flow, they do raise compression, but the overall effects are negative when added to a 350. Any other gen 1 head will fit depending on the size of the bore and the size of the valves, but any good engine builder checks all that anyway, right. The combustion chambers of the 305 heads is significantly smaller than the 350's due to smaller cylinder volume. You can get 49 cc chambers on the Dart Iron Eagle heads, that bumps the 305's compression up alittle and offers significantly better flows on both the intake and exhaust. The only benefit of the 305 is the stroke, which equates to torque. They work ok, but your still down on cubes for the serious HP freak lying hidden in all of us. Will it suffice for a z? Sure, it'll work fine, spank most anything out there and live awhile. But you still have 2 bolt mains so your power potential is somewhat limited.
All in all, though the 305, when properly outfitted can make some decent power, although not as much as the 350 can or anywhere near what the 400 can.