I have a 65 , but it has a 67 chassic. I installed hooker 6130HKR headers. There was no problem installing them. The biggest problem with header is the steering boxs. Then some hit the cross menber on the right. This item was no problem with this headers.
I'm right now trying to build up the money to have it rebuilt. So far as the car goes I've redone front and rear suspension and replaced the frame. The original frame was bad enough that I could twist the back end easily.
Hooker and Hedman both make long tube headers for a '65-68 with an FE, as does Maddog, Cites and FPA. FPA and Sanderson (wouldn't personally recommend the latter) make shorty headers for the '65-68 with an FE.
The Hooker and Hedman headers from what I have heard are hard to install unless the car is on a lift or you are putting them on the engine before it drops in. The Maddog and Crite headers both go under the front crossmember so they can be hard on ground clearance but are easier to install.
I tried a set of Sanderson shorty headers on my '67 and the passenger side header hit the crossmember and I could not install it. Returned those and ordered the set from FPA which are a bit more pricey but seem to have good reviews. Those arrived this week and will be going on tomorrow. From what I can tell eyeballing the FPA headers, the passenger side will fit with no issues.
My '67 had a set of long tube headers on it, but they were not marked and could have been custom made. I pulled them because they were beat to hell from the car being lowered when I bought it. My car isn't going to see the strip, so I figured the shorty headers would be fine enough for me.