It depends on the shape of the manifold flange and the mating flange on the exhaust pipe. Some manifolds will work with either gasket, depending on the exhaust pipe they are mated with. If there's a 45 degree bevel at the manifold outlet, the donut gasket can be used- - - - -assuming there's a corresponding bevel on the exhaust pipe. The donut usually seals better than the flat gasket. If the flange is welded to the pipe, you'll need the flat gasket. Donut gasket systems use a sliding flange and a flared end of the exhaust pipe.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!