well, 5 gallons can fit in the rear trunk nicely, or since you wont be needing that gas tank, theres a whole bunch of space there.. also to make extra power, a turbo and a second alternator is always fun... works nicely too...
And since HHO is a waay better fuel (burns 99.9%- could say 100% but you know...) The motor itself will be more efficient- more horsepower... And also, those worrying about the price of distilled water going up instead... (just a fun-fact..) when HHO is burned, it simply turns right back into water... so if your MK1 is running off purely HHO, you can put your exhaust right back into the "fuel" tanks so youll never even have to fill up!! or if your like me and love the sound of your car.. a small dehumidifier on the front would be pretty sweet... then you can dangle a cup off your exhaust and get a drink if your thirsty... lol
Im not sure why theres a big problem with coming up with the extra electricity required for the electrolysis cells... I know plenty of people with sound systems that require more.... or just think about limousines, 6 tvs, surround sound, laser lights, fog machines, lights, microwaves, fridges etc. or look at some of the hybrids out there- they gain the electricity to power that massive electric motot just by hitting the brakes! Its not too hard too get alot of power from your motor... after all, even the modern day engine isn't even 30% efficient, which means theres ALOT of extra energy there to use...using the HHO that same motor is about 80-90% efficient. these are true facts, im not just blowing smoke here... im just trying to get people to look at it differently.
The only way to know is to test it yourself... math isn't always the answer, sometimes you just have to experiment...