Paint the manifold mounted surface with machinist blue, then scribe the larger diameter using the appropriate gasket as a template.
I use Jamie's (JPNovak) method on a drill press. Moving the manifold by hand relative to a stationary tool is a lot more precise than hand holding a high speed cutter and grinder.
An add'l DIY tip: Use a piece of cardboard the size of your target port size, then run a short length of wire through it for a handle (or equivalent) to help visualize the shape and amount of port material to remove.
Not sure the many hundreds Extrude-Hone charges is worth it for the relatively short length of these intake manifolds.
Sherwood