I have seen big trucks with engine brakes, even the newer 6.7L cummins has an engine brake... so I was thinking making an exhaust brake for the D24.
For an EXHAUST brake you only need a butterfly valve type mechanism in-line with the exhaust system, close the valve for braking action.
seems simple enough,
for those who dont know what this is: here is a step-by-step installation on a cummins 24-valve which will basically be the same thing for the d24.
http://www.youtube.com/watch?v=Tqegu2YKeKY