May I make a couple suggestions? The two outermost corners should probably have a larger radius. As shown the cross sectional area varies from small to large them small again around the outer corners. Or to think of it another way, there is a dead spot/turbulence in those corners. Also, most engines are mounted with a few degrees tilted rearward. As shown, your carb mounting flange or runner shapes do not account for this tilt. Further, stock manifolds flare out at the head flange to go around the head bolt bosses. Perhaps you don't intend to mate to stock heads, I don't know.
Not trying to shoot you down, I appreciate inititative. Just trying to be helpful.
