Just wanted to share my experiences about stainless ebay tri-y headers. They were sold by seller named motor-1 and the item was this: SS Long Tube Tri-Y Exhaust Header Manifold for 64-70 Ford Mustang 5.0 260-302 V8 | eBay
. Headers arrived from US to Finland in about 2 weeks. Price was only 250 usd shipped to front door. Very cheap, so you could already expect some fitting problems. Otherwise, no complaints about the quality, they seem very decent.
About fitting next. Flanges are thick but I gave them some sanding because there was about 0.020" flatness deviance. The inlets of the primary pipes had to be given a lot of beating in order to make room to get the mounting bolts and spark plugs in place. Fortunately, I had some spare heads where the headers could be mocked-up.
The co-driver's header went into place easily, only a small dent had to be punched in the pipe near the shock tower. Instead, more attention was needed on the driver's side. About 1 inch piece of aluminum had to be cut from the bell housing. Small dent was again needed in the pipe near the shock tower. One cut of angle grinder and few degree bending of the primary pipe was needed, so that it doesn't hit the drivers floor. Then a small dent near the power steering cylinder, which allowed me to use the stock PS bracket
After the headers, it was time to fit it to the exhaust system. I decided to cut off the old piping from the front of the H-pipe and connect headers to old piping with short bend pieces and clamps. In hindsight, old exhaust should have been cut much further back, so it would have been easier to make the contacting pipes. Bend pieces and drivers header look like this when a friend made the final welds with a tig.
I was quite satisfied with the end result, even though there was a lot of fitting work. Well, did the rubber start to burn? Butt-dyno tells there's no dramatic change to power, maybe after 3000 RPM there is some more power, but I will visit the dyno to measure it in next summer. You can read about the rest of the engine from my thread: 289 c code port match + mild cam | Vintage Mustang Forums (vintage-mustang.com)
but it's rebuilt 289 with cleaned-up iron heads and small cam. Air-fuel ratios remained approximately the same and there was no need to adjust the carb. The sound is perhaps a little lower tone, but not a big change, no header ticking.