I put TTI long step-tube headers on my 67 Barracuda with the engine/trans in the car, not too long after I got out of a three-month hospital stay and multiple surgeries. At the time my left arm barely worked, so I got a few scratches in the bottom tubes of the headers because I had to drag them across the floor instead of using my left arm to hold them off the floor (front tires were on ramps). But really, I thought they were easy to install. Now, I do have manual steering, but there was plenty of room to work with the motor in place, and everything fit.
The TTI's I put in my 65 Barracuda - now that was a different matter. I still did it with the motor/trans in the car, but I had to remove the torsion bars, steering column and steering gear box, and re-route brake lines and electrical wiring. Plus modify my clutch linkage and dink a couple of header tubes. And modify the driver's side motor mount (original bracket with later biscuit, which wasn't as thick). And probably a few other things I can't remember just now. But even with the small engine compartment of the 65, once the steering gear and torsion bars were out of the way, it was easy to slide the headers in and out, which I had to do a bunch of times before I was done.
But pulling the engine of a 67 Barracuda to install TTI headers? Seem like a lot of unnecessary work to me. Here, the motor is coming out anyway, but I'm not sure putting the headers on the motor before re-installing it would make the job easier. But then I've never tried to do that, so ????