I doubt if you'd obtain any thrust. Even the homebuilt aircraft guys aren't seeing much, due to relatively slower air speeds and lower temps than what the P-51 Mustang operated at.
Having said that, however...
If I ever get a chance to build a tadpole 3 wheeler, I'd put the radiator behind the rear wheel and
hope the heated, expanding air might help 'fill in' the wake behind the vehicle.
Just ideas rollin' around in this melon o' mine.
With the bellypan radiator, like donee pointed out, you'd have to provide airflow at stoplights. I envision the radiator spaced away from the bottom of the vehicle about an inch and an electric blower in the trunk to suck air through the gap.
It wouldn't get as hot as a cat, so I don't think starting fires would be a concern. The heat would be spread out over a larger area.
