Reading your description, sounds like your head pipes & tailpipes have been replaced over the years. How long a case muffler are currently being used? In the early 80's I used to order Sonic Turbo mufflers. They flowed the best & were not annoying sounding, but I was not up against the threshhold of installing them under vehicles whose engines were making 450-500 gross hp. The short case Sonic Turbos worked great under the short bed Chevy pickups & A-body's my buddy's & I were tinkering on at the time. Your mufflers may be something similar.
A bone stock std horsepower '67 GTO would have come with 2" head pipes & 2 1/4" tailpipes. Original 400 HO & RA equipped '67's received 2 1/4" headpipes as well as 2 1/4" tailpipes. Original mufflers were offset side (inlet next to the driveshaft) with offset side (outboard side) outlets. If you are after bone stock type set-up complete with tight bends & flat spots, can either have a longtime muffler shop operator equipped with a card system, hand bend an exact copy of the stock exhaust system made with aluminized steel. The system will not be mandrel bent, will have tight spots in the bends. One can also purchase the same kind of stock bent system from Gardner Exhaust with their proprietary different case length mufflers & clamps, expect a $1000 plus expenditure.. On a high level restoration, that one's willing to give up hp/torque numbers, that is an option. Am beyond anal on my own restorations, but am no way willing to leave on the table, the amt of hp/torque in a restrictive stock kink bent exhaust system.
For many years, have used the 20" case 2 1/2" offset inlet/offset outlet Dynomax mufflers, they are hard to beat free flowing long oval case mufflers. 20 years ago, for my own projects I'd order the 2 1/2" inlet/outlet Dynomax CVX 17205 mufflers (aluminized version of the same size stainless Ultraflow), eventually I started ordering the Dynomax 17449's. For quite a few RA manifolded projects, have started off my own head pipes utilizing mandrel bent "drops" made of 2 1/2" U bend tubing. After the drop, there is a TIG welded seam where the head pipe begins its way rearwards. Basically, the exact same method Pontiac used on it's '73 & '74 455 SuperDuty head pipes. In an effort to have seamless unwelded mandrel bent 2 1/2" head pipes & tailpipes for '70's, 71's, & 72's , a solution is being worked on. One that hopefully many Pontiac owners will like to purchase