I am almost positive that the headers that I installed on my '97 Disco were made of the RRC 3.9 the are made by headman headers you can order them from Summit racing for $109 for the set or a better choice would be to order them from
Jet-hot they ceramic coat them inside and out and warrenty them for life aganist rust...(it also keeps the heat in the engine bay down alot) performace wise I can't give you numbers, but it sure feels better. I would say 15 to 20 horse, again this is a guess.
My invoice from Jet-Hot was $321(US) for HD39800 and $25 for shipping and handling total $346. They are coated inside and out and they say they will never rust through, if they do, they will recoat them for free. If you just want the uncoated version, which you said you do not, you can get them from summit racing for $109.
Hope this helps,
you can also view my gallery and see the headers and what I had to do to make them fit in a Disco (remember they were made for the RRC)