I Kind of did the same thing as you. I bought a cracked H2 and welded it up. Had to make an outside cover and insulate it. I ended up replacing everything on the back with parts not from hardy. Got by a little cheaper that way. I put a PID temperature controller on it and a float and taco valve for auto fill. I run my circulator pump all the time to simplify wiring and cut down on plumbing running to the boiler.
Mine performes fairly well for its size. I heat 1680 sq ft at 75 degrees. 900 sq feet garage at 70 degrees, and 1680 sq ft basement at 72 degrees, and my hot water with a side arm heat exchanger. The basement and garage are radiant in slab heat. the upstairs is heated with a coil in the furnace. On nights that it gets in single digits I get around an eight hour or less burn time but I am putting a pretty big load on it.
Yours ought to work for you. Certainly better than filling that propane tank.