West marine sells aluminum and plastic tanks in a variety of sizes and shapes, including saddle tanks. If you have room forward of your motors on each side, I'm sure they could be mounted with a little glass work.
BTW, forget the electric tank selector valves. I've had a couple different types fail. Use the old fashion mechanical selector valve.
Also, I think there's an article on yachtsurvey.com that talks about correct fuel tank mounting.