please get some header advice 67 dart gt

If your 318 has a stock cam in it and the car is for street use, as in 3.23 gears with a low-stall convertor :
headers will not make much if any noticeable power gain.
This is because the 318 will spend most of it's life at low rpm.
If you are going for balls-out fuel economy, longtubes might make a barely measurable difference....... but you will NEVER in the life of the headers, recover the cost of a TTI/Dougs system.
My advice is put logs on it, and if you can afford them, get the 340 logs with a free-flowing exhaust; duals optional.

But to answer your question
TTIs fit almost perfectly on my SBM*, in my 68 Barracuda, with either trans*, and WITH power steering.
However, I did convert the steering to 73-up, to put the idler arm nut in a slightly better clearance position. But it mighta got into that interference position by my x-tra caster alignment and fixing the bump steer due to my lowered ride height. On the stock alignment, you might not run into that.

SBM*, trans*
I have run both a 360 and a 318 in same chassis, with the same TTIs. While the 360 has always run the A833 M/T, the 318 has also run an A904 auto.