The heater “worked” by passing cooling fan (always running) air over the outside of the exhaust manifolds, IIRC. Hence the leaky exhaust gassing you.
I didn’t get gassed but didn’t get warm either. The same car didn’t have a gas gauge. You had to use something like an oil dipstick to see how much gas you had. It was awful.
But the little bugger was watertight! We had flooded underpasses from a heavy rain. The water came up to the bottom of my windows and the wheels weren’t always in contact with the road, but I kept revving the engine and popping the clutch until we were on the other side of the underpass.
There was a couple sitting on the hood of their stalled Mustang as we went by, sending waves that left the Mustang rocking. It was great.
A week later, the clutch burned completely out. LOL