I was searching for a Merc manifold for a friend, getting tough to find one in either aluminum or c.i. that isn't roached at the gen mount, rusted to hell, or butchered.
I run my 2G without any grinding of the Merc manifold, no problems. I use two gaskets for heat isolation (aluminum manifold) and being a truck, I'm not winding it out to 5,000 RPM, so any flow benefit would be hard to find.