You do not have to open the exhaust ports, the flange was designed for the factory head. The ports in the flange are a shade over 1.625, we did this for anti-reversion. You can open the exhaust ports up on the cylinder head, but it is strictly @ your discression. If so I would need to know because the flanges would have to be modified along with tube diameter. The header as it stands will work best if you leave the exhaust ports alone. If you make the exhaust ports to large, it will kill the exhaust speed and you will not get the benefits from the header as anticipated. To answer your other question Acuta73 for the amount of mods that you have, the Merge collectors will enhance all them.The merges will help in the fact that the exhaust will be scavenged more efficiently, this is crucial when making a camshaft, and intake change which you have. This in turn will make the whole package work in unison, always remember when you do mods on the one side you need to also look at the exhaust side. If the merges are out of budget the formed collectors will work just not as well. If you want every once of power the merges are highly recommended from a performance stand point. Thanks DG