The marine variants are typically high-revving -- not what you need at all.

Phil,
OK, that's good to know, and would certainly rule out the P as well as, of course, the K and the D. The motor I have has a C in it. When you say, The tractor version of the B18 is the B18C. do you mean that they would have used the C cam, or only that they used a B18C type long block, i.e., same head thickness? I see no sense in going with dual carbs, so I thought A cam and the Zenith. Now, if the tractor version uses a cam ground for lower speed and more torque, it would be nice to locate one of those. Then the question would become, what kind of carburetor goes with that cam? Unfortunately, when you experts discuss cam grinds in their infinite number of fascinating variations, I'm usually left sitting here scratching my head. :-(

My experience with marine engines is limited to diesels - a two cylinder Yanmar and a four cylinder Westerbeke version of the Perkins. IIRC, they revved to about 3600 and then used reduction gears to get the props down to a speed where, with their particular pitch, they wouldn't cavitate.
Bob S.