My son and I just installed a set of the Summit headers on his 1988 W100 w/ 318.
These headers are great! There is not AMY fitment issues with the headers! The problem is with people not understanding that certain things need to happen to install them!
They do NOT slide right into place. My son needed to remove his starter, unbolt both of his engine mounts, lift his engine up as far as it would go until the distributor was close to conflicting with the firewall. With that said, it sounds much worse than it was! My son is 17 and a fairly inexperienced wrench (though, he's in his third year of high school auto shop and it's helping him a ton!), and he did this nearly without any help, as I was working on my old '59 pickup and would pop over to his truck as he needed advice. My son installed his headers and reinstalled his engine mounts and starter in approximately 4-5 hours! One regret was not buying new engine mounts prior to starting. They weren't trashed, but obviously original and not in amazing condition. It would have been incredibly convenient to swap in new engine mounts while the engine was raised. I would highly recommend the Summit headers! Great quality and they fit GREAT! they route through some tight real estate nicely! Be prepared to extend the three wires for your o2 sensor about 30", or so, as the factory driver side exhaust manifold holds the o2 sensor. We ran into NAPA and bought an 18mm nut for the new o2 sensor. Before installing the header collector, we used a step drill bit ($8 for a set of them at Harbor Freight) to drill a hole about 5/8", IIRC, then welded the nut to the collector as the new mounting position for the o2 sensor. Worked great! If you don't have the capability to weld a bing in, I know that Summit Racing also sells header collectors that have o2 sensors in them, as I bought a set for my '68 GMC.
ANYHOW, if you realize going into the job that you will need to remove the starter and engine mounts and raise the engine up, you'll realize that the work on the front end saves a TON of time on the total job. Good luck! We installed them on my sons W100 and it sounds great now with the dual 2.5" exhaust and thrush two chamber mufflers. It gets tight behind the headers. We had to jog the drivers side exhaust tubing around the transfer case and to the passenger side of the truck. Looks tough with both mufflers next to each other on passenger side, though. I don't think that you will regret the Summit Racing headers. They're probably made by Flotek, by the sound of it! Summit Racing rebrands some pretty quality parts! I bought a set of Summit Racing SBC Aluminum cylinder heads that turned out to be rebranded Brodix heads! That 383 runs like a ***** ape with those heads, as they came right out of the box!
Good luck!
Nik