Re: First time building
Welcome to the fourm. You've found the source for help with any aspect of building your bike.
If you're handy with metal working you could consider making a new, thicker gasket from aluminum or better yet, copper. Copper gaskets are available from a couple of the vendors here but they are only about 1/8" thick.
Filing the exhaust flange surface flat would also help assure you have a good sealing surface to work with.
Hope these suggestions help but you'll probably get more from other members.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"