For what it's worth I called BIll Heath @ Heath Diesel (the foremost authority on these engines)this mourning and had a long conversation about the GM 6.5TD.
I called to enquire about a propane kit they offered at one time and although it worked well it was discontinued because people would circumvent the safety features that were built in and damage thier engines-then blame the kit,that and when set up properly the fuel savings were offset by propane costs.Power boost was good though.
His experience and advice was to stay far away from propane and HHO-any fuel source introduced pre-combustion chamber is harmfull.
These engines were not designed to survive the pre-ignition that can be involved under certain conditions.
The advice given was to go only with water injection-you get cooler egt's and the combustion temperature of 3000+ degrees actually creats HHO in the cylinder not unlike the way nitrous oxide works.
His kit was designed to use tap water at a flow rate equal to the fuel consumption.60 gallon fuel tank-60gallon water tank.
That is much higher than other kits propose using but the R+D done proved it the best soloution and beyond cooler EGT's adds 1-3 mpg on average depending on the load.
Bill even put the kybosch on the after cooler I intended to install.Air flow restriction that is preset on the heat exchanger is detrimental for these engines.
So I guess the point is that these 6.5TD's can generate HHO internally and that is the safest way to do it.Economical too!