What was the original carb size used on the Merc 350/260 hp motor?

Well, depends on what year. The Rochester Quadrajet was used for a long time, but that won't answer your question. The Q-jet is an air valve, which means that it flows only what the engine wants, up to a max of 750 cfm (the 850cfm Q-jet was not used on the marine SBC to my knowledge). If you are looking for a Holley or some such, I would stick with 750 at the low end. Chevy Hi-performance Mag had an interesting article recently on carb size on a SBC and you'd be surprised to see how well larger carbs did. Those engines weren't torquey marine engines though.

750CFM....Though on a stock SBC 600CFM is fine. Almost all Merc 350/260's came with 750 Quadrajets. I think a few were made with Webers. Not sure about their CFM's though....If your Quadrajet is working o.k. And your not pushing more than 400hp leave it on!!!!

BH