Correct on no gasket needed for the crossover pipe flange on Olds 307 V8 with stock exh manifolds.
However, the gaskets for the exhaust manifolds are discontinued by GM, but you can get aftermarkets, or find NOS ones on the open market somewhere.
The donut gasket is for the down pipe to the cat. That's the one with the 3 bolt/spring flange on it. The EFE valve on driver side has a 2 bolt flange gasket between the manifold and EFE valve itself.
GM p/n 22526988, FELPRO 60771 or equivalent is the EFE to manifold gasket on the driver side.
GM p/n 1262500, Walker 31511, or Mahle F7207 is the donut gasket on the passenger side to the down pipe.