Be advised that a 3" system will also be LOUDER than a 2-1/2. I wasn't expecting it, but having done one of each the difference is obvious- the 2-1/2 sounds healthy, the 3" sounds like it wants to eat your children.

I put a 3" on my 71 cuda 440 with stock manifolds. It all went in fine, but had a horrible drone at 45 mph. I installed their headers, and the drone was gone. It was the easiest exhaust I have ever installed.

I had that exact same experience with my 1970 'Cuda. Manifolds with 2 1/2" pipes, terrible drone. Put TTI headers on, no drone and more power, a winning combination.

Yeah, I had replaced a tiny exhaust with the 3", but left the manifolds. It woke the car up, but after installing the headers it was a whole nother beast!

Wondering how the TTI system would fit using Doug's Headers ??

Also have the rear springs moved inboard so the pipes would have to run off to the sides of the frame rails - is there adjustment there to move them closer to the tank? (69 B-body)

I have the same question, I was going to email TTi. I found out Doug‘s headers are on sale at Summit Racing and you can use a 10% off code on top of that makes it a sweet deal. AutoZone online has a 20% off sale so it equals out to be the same price as Summit’s. They both have free shipping.

The X-pipes have slip joints to allow adjusting the side-to-side spacing to fit the header collectors, so they're not header-specific. But that's the only adjustment in the system that I can think of. I'd contact tti to verify.

I put a 68-70 2.5"tti system on my 67 coronet. I moved the leaf springs to the frame as I mini tubbed it. It fit perfectly. The catch is that I removed the gastank and spare tire floor.
I do have a 20gal fuel cell that is sunk down through the flat trunk floor that I installed. I see what I can find for a picture.

Interesting that some kind of sound wave cancellation is going on at highway cruise rpm
when individual cylinders get the added length of their runner down to the 4-into-1 merge, whether 2.5 or 3.0 collector.

https://ttiexhaust.com/Menu_Classics440_Headers.htm

https://ttiexhaust.com/Classics-Hdr/383440-134/TTI383440-134LS.jpg

TTI does not want to say the lengths of the runners so competitors can easily copy.

The runners do not look equal length.
Overall width 41 to 42 inches

Longest runner I guess-ti-mate less than 42 after subtracting collector length.

I wonder if TTI scientifically experimented with varying runner lengths to get rid of part throttle drone in the 60 to 75 mph cruise range,
or was it a “happy accident.”

Sales volume wise having a reputation for “no drone” probably is more valuable than
“more max rpm power gain” than any other header.

Doug’s header design emphasis on “true equal length” would have more chance of sound cancellation just at one relatively narrow rpm and throttle position.

I think the drone is caused by factory manifolds. I had a 68 charger with a 360 engine with stock manifolds and a summit 2.5" dual exhaust system. It had an awful drone in the 40-50mph area. Maybe a bit wider. The 2.94 gears made the drone range wider.
Ultimately what fixed the drone was installing a cheap set of Summit headers.