It's best to buy the manifold that best matches the size of your runners. That way the whole manifold will be sized properly. Then as recommended above, do the port matching routine. And high speed grinders work great w/ carbide bits on aluminum if you use grinder's grease that totally prevents sticky aluminum clogging of the teeth Eastwood
sells it. I have a tube that has polished three tranny cases, several intake manifolds, etc., etc., and still has many projects left. Probably a lifetime supply for a hobbyist.