I would check out both TTI and Accurate for what they offer for fitting to your Hooker headers and your mufflers (if you are keeping them). Your request is just a bit different than what either typically make, which are systems to fit their headers and mufflers or stock. You will need to get some dimensions. The length of the muffler, the in/out offset if any and the width and height. Also look at what type of hangers each offers vs. stock. The position of your collector flanges will be the harderst to measure, and will only be a good fit if TTI or Accurate has used the exact same headers for a mock up. This portion you may want to have a semi-local shop make up or fit. Go to a car event where there are street rods and ask where the ones with nice workmanship were done.

Sizing. Going to a larger tube will slow velocity and let atmospheric pressure move up from the tailpipe at low rpm. For street use, keeping velocity up at less than 3000 rpm is more important than what happens at 5000 rpm. However a good balance is achievable.

The exhaust gas takes up less volume as it cools. Just because a 3" collector diameter is the right size for high rpm performance, doesn't mean the whole system needs to be 3".

For a street car with some interest in occassional 1/4 mile performance, consider whatever is attached to the collector as an extension of the collector. Wherever the size drops, or there is an interuption with an H or X, that's the effective collector length. You can change the sound reduction and help or hurt power in certain rpm ranges by where the H, X or drop in size is placed.

You can use Pipemax software to estimate best placement. It will be most accurate when adjusted with specific dyno data. But I can tell you that for a close to stock engine Pipemax says the collector should end around 20" from the end of the primary pipes. Next best would be around 40" and third best around 80".

My experience and observations with A-bodies has been that TTI systems use stock tail hangers but not the muffler hangers. Whereas Accurate uses the stock style muffler hangers but nut not always the stock style tail hangers.