Hey LM,
I know I would be definitely be under 18 gallons for a west side of Possession four or five hour trip.
Even though I'm running twins, my total cubic inch displacement is only 400 (Evinrude 200HO's are 200 cubes each) compared to your 305 cubes.
My rig cruises at 34mph so I'm guessing that those twins are running for less time than your four stroke I/O.
I really don't think that changing the carburator will reduce your fuel consumption but if it's been a while since a carb service you might consider a rebuild. I don't know if Joe's Carburators in Lynnwood is still in business, but if they are, that's the place I would take a marine carburator. They did two carbs for me when I was running I/O's and the engines never ran and idled better!
Speaking of Inboard/Outboards, I can't tell you how much more maintenance they are than an outboard motor.
Think about it for a minute: the four stroke automotive engine with it's oil pan and starter occassionally soaking in a saltwater bilge which leads to rusted, corroded parts and electrical connections. The same saltwater splashing around the sparkplugs, wires and distributor stressing those components shortening their service life, dependability and leaking electricity everywhere resulting in dead batteries. Been there, done that.....and I'm done with that! No more I/O's for this guy!
I'm a Stabicraft aluminum boat, Evinrude Outboard guy....since you asked