Propane Powered Engines Featuring a 7.3 Ford and 6.7 Cummins

Yawn.... Check out the energy capacity of propane vs Gasoline or Diesel. You's be cutting your driving range to a fraction of what it is currently. Or hauling around a massive and heavy, large propane tank. This as pointed out above is old tech and usable by short range vehicles only. Propane does burn very cleanly and probably improve engine life but you will pay for it.
Same applies to hydrogen plus adding hydrogen's many other problems.