In 1999, I bought/installed the TTI 3" system on my 68 Barracuda, I ordered it with the turn downs. At that time it came for the longer Dart wheelbase.
I started the installation from the back, putting the turn-downs where I thought they should be, then, and fabricating new hangers as I went. In my car, I only had to bend up one corner of the fueltank, up near the shocks, which were old Munro, three-way adjustable HD types, which were a lil fatter than the ones in the above pics.
At the front end of these tailpipes, I used the factory brackets that are welded/bolted to the floor somewhere close to the seat-belt anchors. I shifted the monster hangers this way and that, to maximize the clearance.
Then I moved to the front.
First I installed the mufflers and positioned them on blocking. Then;
As expected, the pipes were too long
I bolted on the collector extensions, then one by one, measured and cut the pipes for about a 2>2.5 inches overlap at each end; the fit was perfect.
I did not clamp any pipes together, they are all slip-fit.
The H-section keeps the front aligned, and after driving for a few hours, the two pipes soot up at the tight-fitting overlap and they don't move.
At the mufflers, I drilled small holes thu the overlaps and put a screw in there to keep the muffler anchored in place, and not spinning. At the back of the mufflers is where the hangers are, just snug enough so that the U-clamp nuts do not vibrate loose.
After the tailpipe fit was finalized, I again drilled holes thru the overlaps, with screws following.
And that is how it has stayed ever since.
To get the pipes apart, after the screws are out, I have to spray penetrant into the slip-fits and wait. In a couple of minutes, the soot relaxes, and it comes apart just like it was new.
In 2004, I fit a GVod to my A833, which required the removal of the H-pipe.
A short time later my 223/230/110 cam lost lobes and had to be swapped.
From the same manufacturer, I then bought the next size bigger; 230/237/110.
I immediately lost a bunch of low-rpm torque. But the sound out the turn-downs changed gloriously. So to get the bottom end back I installed a Commando box with 16% more first-gear torque-multiplication; and the H-pipe never went back in. And so it has remained to this day.
Having said that, dual 3s is obviously more pipe than my 367 needs. So the Pypes option mentioned above, with 2.5 after the mufflers, seems reasonable.
In my case tho, even with the springs offset inboard some 5/8ths inch, with the Mopar hangers, there was plenty of room for the 3 inchers.
But I have heard the stories.
You won't believe the wonderful sound of dual 3s at 7200, splitting gears ..........
The turn-downs were/are, just right.