Skippy, the gas heaters only use 3 ounces or so per hour. So that is nowhere near enough to worry about, wherever you tap into the fuel. I tried to get that point through to Mr. Drake, but he put in a separate small lawnmower type fuel tank anyway. A simple T with a small takeoff for the heater will completely suffice.
For years I had an old Eberspacher BN-2 like Gordon has. That went to Cory Drake. It came out of my old car.
For the new car, I purchased a new Webasto Airtop. That thing is tiny, only a foot long and 5" square. It fits almost anywhere in a replica. I paid about $1000 but you can sometimes find them for around $700 new. I WOULD NOT BUY A RUSSIAN OR CZECH KNOCKOFF. Be careful, most run on diesel. A gasoline(benzin) model costs more, but it simplifies things with fuel, at least for me. It is the cat's meow.
Also, carbon fiber electric seat heaters are pretty inexpensive. The labor to put them in costs more. But they work quite nicely, especially if they are in the base and seatback. We did our own, Michelle and I. It wasn't too hard really.
For the record, I may have the only Spyder that has seat heaters, heat and actual working windshield defrost vents.
And I don't care what Jim Ignacio wears or uses for heat. He's a cool dude to me no matter what anyone says.