Paul is right.
An uncontrolled heat source into a pressurized tank is a potential bomb. When you get a steam explosion, it usually demolishes the entire building. It is generally quoted that one gallon of superheated steam has the explosive potential of one stick of dynamite. Ok, you're going 40 gallons or maybe bigger, so fifty sticks of dynamite with a twelve dollar insurance policy (temp/pres valve). An atmospheric system wouldn't be too hard to rig up, which is inherently safer.
Having said all that, I'm the guy that's running a Lister cogen setup that's also an unregulated heat source with not one, but two, converted gas water heaters. So I'm breaking the law just like you are. At the very least, I'd run redundant pop off valves, which is what I have. I also keep a very close eye on my water temps, which have never gone above 135F. The coolant loop of the heat exchanger only holds 3 gallons of non-tox antifreeze, and it stays reliably at 190F, but that part of the system is atmospheric, not pressurized.
Good luck, have fun, play safe!
troy