I am doing duals on my 78 GP SJ with a 301 right now. It had the original exhaust all the way back to the factory dual outlet muffler. First, yes you can use a third or 4 gen muffler but it isn't easy or exceptionally clean. The muffler inlet (drivers side for this car) and driver's tailpipe are flipped on the f body muffler vs the factory g body dual outlet because the f body mounts in a different orientation. A muffler shop could overcome this but if you are trying to do it with auto parts store stuff it's not so easy. The f body tail pipes are also larger and won't fit so you have to make them or modify/adapt the smaller gbody stuff. This is not a bad route to go. I will warn you that the f body stuff is stupid quiet on a g body. So quiet people commented on it to me. That sums up the dual outlet f body exhaust. Which wasn't really your question.
It's been stated correctly there is no factory dual exhaust for this car. But you obviously want duals, and the factory y pipe and exhaust are a whopping 2 inches AND has more kinks than R Kelly. This is my answer to that problem.
I am using a dual system from a SBC car that is hedder back. So that gets you over the axles and tailpipes. From the motor you need to be creative. First do you want to retain the heat riser on the driver's side? Are you OK with just leaving it in and stuck open? If not you need a drivers manifold from a 68-72 A body (there may be others but that's what I had). It is rear exit. Most Pontiacs are center exit. Maybe you can make that work I didn't want to try it, especially with my clutch linkage. Next the 301 uses the triangular 3 bolt flange. No one offers those (that I can find). So order 2 walker 41475 exhaust adapters from your favorite auto parts supplier. They come with 2 bolt flanges. Cut the pipe in half in the middle and slide them off. Save them for another job. Move your original flanges over. Weld the exhaust adapter back together. Now you have the correct flanges and a 2 1/4 exhaust stub. Fabricate from there using the appropriate parts from your favorite source to connect to the SBC system you are using. I recommend starting from the rear and working forward.
An alternative to cutting the pipe in half to change flanges is to cut the 2 bolt flange off and get a repair 3 piece flange. Autone has them. They look like a box of Lincoln logs but it will work. They aren't cheap either.
Good luck. If you come up with a better way fill me in. I hate doing exhaust. Love shirt cuts and cheat codes