My folks put one of this mason jar things are there Chevy Gasser V8 van. They get about 2mpg improvement. I have seen the things work, and they do work, just not as well as advertised. To be useful on a 7.3 you would need several of them. I have seen pictures of them hooked in series.
I wont be putting one on my truck until there is further testing by someone repetable as to long term issues. I would like to know if the HHO will leave any kind of deposits, or will it screw around with the sensors, ect.