You are going to have to go with a after market carb. Demon makes a great one.
Best thing to do would be punch out the engine and get bigger pistons. But if you want to stay stock size then just have the heads ported up to match a new dual plain manifold with the demon carb. And you can put in a cam for what your looking for. Low end, or high end or midrange. With those you should be close to 350 engine horses. The drivetrain will have to be modified if you want 350 at the wheels tho