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Hedman hedders 89140's will fit but require extensive use of a hammer to make fit. If you have power steering the driver side won't fit without modifing the 3rd primary tube, it will have to be cut and rebent to fit. The passenger side will fit by banging in the side of the collector about 1/2". I have the headmans on the 67 in my gallery. If you can afford them I would recomend L&L headers they are a little pricy but they are well worth it.
I find it strange that the regular '73 - '79 headers would not work, as my '68 used the '73 - '79 motor mounts to bolt in the 460. So it would appear that the frames are the same. Is it possible that installation of a later 460 into a '68 F350 makes no difference. It also might be that the people that are having problems installed a 460 into a F100. Just my thoughts.
I just went to the L&L website, and they list the headers for all 2wd F250s and F350s with a 460 as the same part number for '67 through '79. I could not find a listing for 460 headers for 2wd F100s and F150s, but it looks like there is a typo in the website. Regardless, they would be a different part number. I think this is where some people run into trouble. They try to install a 460 into a pre- '73 F100 and the F250 headers do not fit.
I think I'm going to try the Headmans, and I'll repost after they are installed.