I tried the Summit 3" aluminized steel exhaust for my 64 and man, do NOT try. Spend a little more and get the flowmaster setup. My pipes hang down in full view and I could not even get the over axle / tailpipes to come close to fitting. As stated above the summit mufflers are really loud. I started with the full welded turbo mufflers, do not recommend, then went with their regular turbo mufflers, still loud. It does sound like a muscle car, but I'm sure my neighbors hate me. Any WOT runs and your ears ring afterwards. That's my stupid trophy for going 3" and being cheap with mufflers.
Ended up spending $150 more and getting flowmaster tailpipes, they fit a lot better. You can see how Summit just got the stock pipe routing and put 3" pipe in the bender. It hits the trunk floor, upper arms, diff cover if you're running an aftermarket stud girdle, springs, AND hangs under the gas tank.
The 2.5" may be a little more forgiving? No amount of trimming would bring it in to work. Flowmaster tailpipe on left, you can see the height difference is dramatic.
You can see how the pipes from headers back hang below the frame, this is with the summit turbo mufflers.
This is the fully welded turbo mufflers, they're extremely "clappy", you can hear the exhaust echoing off of houses, this was about 3/8 throttle.
The fully welded sounded awesome with turndowns though, but again, hearing loss is guaranteed.