with your exhaust, if you can get a new flange that suits the orig manifold, upsize the main engine pipe to 2" with a single sports muffler. if you cant get a new flange have the exhaust cut a couple inches below the orig flange and go from there, the A series engines love to have lots of air go in and out the faster you can get this happening the faster you can get the engine to go.
even with the stock carby you'll notice a big improvement in the rev range