Re: Chevy Exhaust setups.
Looks to me like the right side is made for a FLAT gasket, even though the inside is chamferred a little. The headpipe flange must also be flat. To get flat flanges to seal, you have to loosen the rest of the pipe so the flat flanges can line up without trying to bend or twist before tightening.
But - the part you removed - the heat riser - the cast thing you tossed away - it's designed with both ends different so you can't toss it without changing the headpipe flange.
Get another one with the proper gaskets for it and it'll work fine. They usually are flat flanges on top at the manifold, and flared on the bottom.
No matter what type of gasket you use it's not going to seal when you mate different types.
Nuts - best to use brass nuts, available at any muffler shop. They don't rust on.