A customer of mine used an old Caddy radiator to heat the garage where his coal boiler was. He used an old hanging gas heater whose exchanger was done for and tossed everything but the end with the fan and the front. He just fabricated a sleeve that fit around the radiator and sandwiched the ends together. It would turn on and bring his garage to temp in a minute or less.
If you think about it , car radiators dissapate alot of heat. And you can get 30lb radiator caps. He used clear tubing and stainless clamps to go from the radiator to the pex. Necessity is the mother of invention. I should go visit with a camera. It really was a good Idea.