My TTIs did NOT bolt to the seat-belt plates.
There was a proper hanger already on the one side, and I bought a matching hanger for the other side, from Year-one I think. As you said; further back and higher.
Get the pipes hung from back to front with the mufflers
before you start cutting stuff.
The pipes I bought were for the same as yours and they fit on the 111wb cars as is, mostly. But on the 108wb cars you have to cut to fit. I had to cut about 8 inches off the front to fit the TTI longtubes on my 68 Barracuda. Mine looked sorta like this
View attachment 1715760340
or this
View attachment 1715760338