As I mentioned, I am running the JBA 6611 headers on my 68GT350. I had the Hookers on there and the ground clearance was too low.
The collectors on the Hookers point down and start at a lower reference point then the JBA's do to begin with.
The Hookers are 1-1/2 primary tubes and the JBA's are 1-3/4. The JBA's will fit without the ps extension but the rubber accordion boot will be up against the #5 tube.
With manual steering there is no issue with steering components at all with either the Hooker or the JBA headers.
The difference in the size of the tube diameter is worth 1/10 in the quarter mile which rough translates to 75-80hp gain over the Hookers.
The location of the collectors and the angle of them on the JBA's is MUCH better then the Hookers. They are tucked up into the chassis and will not have any ground contact issues themselves whereas the Hookers will.
My car has a lowered suspension and with the Hookers, you could not make a fist and slide it under the collectors.
It's irrelevant what will fit on a 65-66, or a 69-70. They are SIMILAR chassis but vary enough to not be an exact comparison of details, as well as how a Boss 302's or a FE fits from the same manufacturer. What matters is what fits with a 289-302 in a 67-8 chassis. Both will fit. The Jba's fit much better, even with the bigger tube.
IF you are running stock displacement, heads, cam and induction, the bigger tubes won't do anything for you.
I have run hp iron manifolds also. In fact I even tried iron 351w 4v manifolds which are larger in diameter then the 289hp manifolds and have the same configuration. They are a waste of time. They are giving away at least 30hp and make the engine feel small and rinky-dink.
Tri-y's are not what you want to run on these cars. They were a compromise on the 65-6's and were put on there for easier installation as much as anything back in the day. They are not much better then the iron manifolds are and give away almost as much potential and power.
That's pretty much what I know about the subject. *subject to revision.