Rather put the exhaust energy into thermal fluid, then use a Heatgate to get said thermal fluid cycled into pressure area of Stirling engine. The efficiencies will be many multiples of where you were aiming.
The ability of a Heatgate to bypass conduction in such matters is cutting edge with Stirling now capable of in excess of 500 Bar.
Lead or follow. Using the radiator low grade heat is going to waste your time in comparison.