I have to agree with bob on the headers based on your motor trans and gears and mostly how you describe your driving habits. I have installed many full length headers in 1st gens and it is not a job to go into lightly. I have never installed with factory a/c but own a stock 68 350 a/c vert now and would not wish that job on my worse enemy. Bob is right in my opinion on the long branch manifolds being a better option for your application and I'm pretty sure you have the end holes drilled/tapped in your heads. But just take a look at the ends of one of the manifolds it will either be an empty hole or have a bolt head peaking at you. My advice to you would be to put on one of the stock oe dual systems that Ames offers. I have made many dual exhaust on a pipe bender over the years and the problems you will run into with a lot of the over the counter high performance exhaust systems is they will not have the jogs in the pipes like in front of the dual mufflers and will instead just be straight. This is part of the reason these generic systems do not fit close to the body like original systems and having a vert. with the brace and stock slightly sagged suspension can cause lot of fiddling and rattles as well as ground clearance issues. One other thing to consider is noise, most of us love a nice throaty bark from our exhaust note but the unintended consequence of the mufflers your looking at and a lot of others is a terrible drone noise that will drill a hole in your head while cruising at steady rpms and can ruin the cruising experience. The stock dual oe with resonators will not give the drone and has a decent growl when accelerating. You have to think of an engine as an air pump, having to free of an exhaust system based on displacement heads intake carb cam and rpms is a total waste and will do nothing for you but make you feel better. Also if you go the stock dual route I would leave your manifolds alone and if you want to upgrade later any good shop can adapt your new setup to your system. It's my opinion with your 326 and specs. this would give you the cleanest best fitting with no drone setup