Do you have an idea of what kind of MPG you'll get out of the new inline 6 cyl engine? Would it be comparable to Max's fuel consumption since it's also an inline 6? A couple other key points to consider when trying to compare them are the boat and engine configurations. Your Morgan & Max combo is a high HP/weight ratio low drag hull, like a gas thirsty big block muscle car. Fuggly with her new engine will have a similar engine to Max, but with a much heavier and more draggy hull. Comparing Fuggly to a big block RV motorhome would be appropriate in this case IMHO.
I've always heard when you're building something like a barn or a garage to build it bigger than you think you'll need. I would think fuel capacity would possibly follow a similar thought process. The exception to that would be a race boat where you only want to have enough fuel onboard to finish your race. On a pleasure craft like Fuggly you're really just looking to fuel it up so you can enjoy your time on the water. As you've seen with Miss Morgan, having to come in to fuel up takes away from that time, for you and your passengers. I agree with EZ, go as large of a tank as you can. Once you've had her out on the water a few times you'll get an idea of how thirsty Fuggly will be. Once you have that figured out, then just fill it up with as much as you think you need for your trip with a little extra for reserve. You can always put in less fuel in a big tank, but trying to put in more than a small tank can hold will just make a mess in the boat.
FWIW, I'm thinking of putting dual tanks in the Laker, one in each sponson. I'm not going to put the details in my build thread until I get a few more things worked out in my head first.
BTW: I'd go with a swim platform with flip down ladders. Fuggly just seems like a good match for one with how I picture her being used in my head. As far as the materials for the platform and ladders go, I'd try to keep it retro looking so it would go well with the style and color of the boat. I'm not sure what kind of accessories were around and popular when Fuggly was built, so I'm not sure what materials to use. Might want to wait to put it on until after you get her splashed her first though. That way you can see where your water line will lay with all the changes and repairs you've made to the hull and engine.