Diesel-Propane dual fuel systems can work well but the low-tech systems are very risky. Propane's critical compression ratio is ~12:1 and its use must reduced as engine loads increase or detonation will result (see Dual Fuel Fumigation). I expect that Swamps Diesel Performance has had experience with the low-tech systems. A diesel-propane system that uses a fuel map to control propane supply is much safer and the only two that I'm aware of that have a fuel map are the EcoDiesel System and the Diesel Magnum.

Diesel Dual Fuel Conversions