I'm going to assume you have an '84 camaro. If you want true duals you WILL have ground clearance issues. Unless you notch the subframe. Collectors with turndowns would be pointless, obnoxious, illegal, and you would keep a migrain.
If you want a good sounding, loud car, with unrestrictive exhaust, then get header reduces, probably takes 3 bolts to hold them on. Then notch the subframes and weld a plate back on the bottom to reinforce them. Then run the pipe back thrue atleast a converter shell for the visual inspection, and then into your flows. Either come straight out the back or turn-downs at the rear-end. That would be your best bet.
Soundwaves are amplifyed by the flow of exhaust. Do not run a huge pipe, and yes 3" is too big. I would go absolutely no larger than 2.5" pipe with the full exhaust like I was saying.