340 Exhaust Manifold Help

HELP! I have a 340 motor with magnum heads and I want to put my 69 340 manifolds on it. They bolt up just fine,but the port size mismatch will cause a huge exhaust leak. The magnum ports are about a 1/2 " smaller in height and causes a gap on the bottom of the exhaust manifold. Has anybody else ran into this, I can't believe I'm the first one.