I bet it's longer than you think. How much is a gallon of distilled water in Texas. In MI, it's about $2.19, only about $1.00 cheaper than a gallon of gas. Considering an inital investment of $3,000, that's a hell of a lot of water you have to run to be able to break even on the change.
Especially, being an experimental engine, even given that your husband is a mechanic, there will still be considerable cost in parts and labor.
I'm not trying to discourage you on the deal, just trying to make a point, that trying to go green to save money, doesn't always save as much as you would like, or think.
My car is a flex fuel vehicle, I'd love to run E85 in it all of the time, but my gas mileage on E85 is about 15 - 20% less than on regular gas (the energy output of ethanol is less than gasoline). Since E85 doesn't cost 15 - 20% less (until gas reaches about $4.50), I'll continue to run gasoline.