I tried putting HMMWV headers on my M1008 - right one hits the firewall (can be cut and a quick 45 degree to clear), and the left one angles out and hits the frame (looked like major surgery would be required, cutting it all apart and rewelding at a different angle). I quit trying at that point since I got a set of used long tube headers (brand unknown). The left fit fine, but had major issues getting the right to fit and it still rubs the frame after some reshaping work. Took nearly a day just to install the right header.
No noticable change in mileage after doing the work (and putting on dual 3" to mufflers, dual 2.5" mufflers & back). My $.02 is it is definitely not worth the effort.