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What brand and where did you get them, right now I am running a hooker comp on my 78 F150 400 auto and dont like how the come down so close to the crossmember. Also they are not even side to side on the outlet of the header.
Take a look at my gallery under "Headers 101" Teses are the best fitting set of headers I've found for the 351M/400 engine in the 4x4 truck chassis. They are made By Sanderson out in Calif. and mine are ceramic coated. I'me been running them for the summer now and they still look the same as when I intsalled them. One final word, they are not cheap.
I tried hedman's full length inside the frame headers. They went in ok but I'm running 2.5" duals, no way was I going to clear the crossmember the way they were set up. So yanked them off (and now they are in my garage collecting dust w/ less than 5 miles on them) and put on hedman's fenderwell exit headers. MUCH BETTER. Install was fairly easy (harder than inside frame which were drop ins, had to shorten a cab mount bolt & bend both fender aprons a bit along with oil dipstick tube & brakelines) for the headers but a ton easier to hook up the exhaust. I know not everyone likes having the exhaust on the outside of the truck for the whole world to see, but I don't mind the way it looks & it's good not to have all that heat next to the tranny. The only thing I didn't like about both sets is they come really close to the power steering pressure line - although it might be because it's an aftermarket line and not OEM, I had trouble with its fitment before headers. Bought both sets from summit. If you want inside frame headers ebay always seems to have nice sets for cheap.
I vote for the fenderwell exit headers too. You do have to move a little bit of stuff around but it's well worth it. I'm running L&L fenderwell headers and love them. I'll never go back to a regular inside the rails exhaust.