Yes, you'll need to blend the exhaust manifold ports to match the head ports. Even a relatively abrupt cone shape in the manifold to prevent turbulence as the exhaust gases enter it is better than creating a step. Those round ports are intended to be used on a race-only engine with headers of the same size and shape as the ports. As I've been preaching for years, doing race engine stuff on a street engine makes no sense, and usually ends up with a very temperamental engine which only develops increased power at RPM ranges and road speeds that makes the vehicle a ticket magnet. Cops and judges love hotrodders!
Jerry
