On my way back from the post office today, I stopped at my shop and threw a caliper on a Merc manifold and a 2G, just to see how much difference there was. The throttle bores in the Merc Manifold were 1.300", while the 2G has 1.4375" throttle bores. A little basic math and I came out with a total throttle bore area of 2.654 sq in for the stock Merc setup and 3.418 sq in for the 2G. That means the throttle bore area of a 2g is almost 30% larger than the Merc. Definitely a significant improvement.
Of course, this means that, although the 2G will bolt to the Merc manifold, it won't do a lot of good unless you bore the manifold out to the 2G size. I have done this on both aluminum (Canadian) and cast iron manifolds without a problem. I used a scrap 2G base as a guide and a tool called a "Rota-Broach" in my $89.95 Northern Tool benchtop drill press. The cast iron cut easily, not to mention the aluminum.
Then again, if you want to get the cheapest performance increase from a flathead, have the heads milled for increased compression and optimum quench. This is not just a plain mill job, but requires some measuring before to determine the optimum cut (which may not be the same on each side) and some work with a die grinder after to get uniform clearance over the piston. Valve to head clearance is not a problem with mild cams.
