The best headers I've used on a SBC were the Headman Elites. They use ball and socket arrangement on the collector and no collector gaskets. Use top lock all steel lock nuts and you should be good. They may need a little re tightening after a few miles, but should settle in pretty quick.

The flange gaskets I've had the best luck with are the multi-layered aluminum ones. Mr Gasket made the ones I've used, but there are others. These have a sort of honeycomb construction and can deform to seal very well, at least on the engines I used them on. They will need to be retightened a few times, but once they take a set, I rarely had them leak.

Considering how bad those 305 exhaust manifolds were, even shorty headers would be an improvement.