I have heard that the short tube headers are not that much of an improvement over stock. These being the edelbrock, BBK, and JBA. However, I have heard that the long tube headers do improve the numbers. The only version of a long tube header i have seen are the Dyna Tech Headers. These are the set of headers that i choose to go with and I have been happy with them. They also have high flow cats. as an option. I have them but i have also melted a set. Next I am going to bypass O2 sensors and drill them out.
Hope this helps.
B
all exhaust
http://www.summitracing.com/search/year/2004/make/HUMMER/model/H2/Department/Exhaust/Engine-Size/6-0L-364/?Ns=Rank|Asc
Dyna Tech
http://www.summitracing.com/parts/DT...DTC-115-841350