I am also partial to a 2GC on a Mercury manifold. However, as Ron has said it is important to bore it to match the 2GC throttle bores. I use a scrap 2GC base as a guide and a tool called a "Rota-Broach" to do this in my drill press. I have done it several times with great success, on both cast iron and aluminum manifolds.
Here's a link to a post I made on the H.A.M.B. with more detail :
https://www.jalopyjournal.com/forum/...#post-13738492.