Out of interest, how is your crankcase vent set up? I have a standard Reniflard unit venting to air. Your smell is quite a puzzle!The exhaust is not leaking at the manifold exit. I sealed the area above the radiator and around the top of the right side cardboard. I sealed the heater core to the cardboard. I also slowed down the fan (adjusted my voltage convertor) since I was running it a good bit faster than it would be on 6v. It might be a little better, but I am not sure. It has been unseasonably warm here recently, but I will get a better test next week when it will be cold. I might also seal the top of the cardboard on the left. to keep a bit less engine compartment smell from the radiator intake. It would be interesting to know exactly what the airflow does in the engine compartment. I suspect that air circles around the edge of the radiator anywhere it can.
The heater duct is new - I had a rubber duct, but changed it to aluminum just in case the rubber was the problem. It did not make a difference. I also need to make sure there is not any petrol seeping out on the top of the gas tank. There are no drips or stains on the side of the tank, but the tiniest amount would make the heater smell terrible.