I talked with TTI before getting my system and they basically said the switch over from 2.5 to 3 inch is at about 500 HP. I have TTI headers on my Cuda 340 and hedman headers on my challenger 360 (original 318 car), bot are 1971 models. I have TTI x-pipe on both cars, 3" on the cuda, 2.5" on the challenger. I wouldn't say either exhaust system was perfect but they went together without much trouble... i assume any difficulties were probably due to variances on the cars since they're in their 40s as far as age. As for out of the box pre-fab systems I do recommend the TTI ones, they're very nice.
On the challenger i have a borgeson power steering box and hedman 78056 headers. I found that the stud (dunno the technical term for it) that connects the pitman arm to the center link hits the left header. I basically beat a dent into the header to get clearance (yeah, i know).
On the cuda i have the newer style TTI headers that do NOT go under the steering center link. I had to make new trans cooler lines as the stock ones wouldn't fit right. I also had to wrap the starter in heat shield.
I have flowmaster super 10s on the cuda and flowmaster 40 delta flows on the challenger... (yeah i like flowmaster, i even have flowmaster exhaust on my Ram 1500). I like the sound and the performance.
All in all, despite the mild quirks, i'd definitely recommend TTI. If i had it to do over again, i'd have done it all the same except getting TTI headers for the challenger instead of hedmans. Nothing wrong with hedmans, i've used them for years, but i find the TTI looks better and the fact you don't have to drop the centerlink to install/remove is a big plus.
Just wanted to share my experience and hopefully it's somewhat useful