Exhaust Suggestions / Assistance

Just some suggestions from experience with almost the exact setup in the past.

For decent power and performance out of that 318 you will need a bigger carb than a 500cfm (up to 750 maybe) and a 2.5 exhaust off the headers can support around 500hp so I think you will be fine with that, plus there is a fair difference between 2.5 and 3 as far as routing room under the car.

On the exits, H or X in the pipe isn't going to matter a lot, so whichever is easiest or whatever will be fine, but for noise sake you probably want one or the other.
As far as mufflers, this may be where to want to drop some cash.
I wanted the old school Flowmaster sound, but you may consider something like Borla that are a little milder tones but still a bit mean sounding.

Turn downs under the car are loud inside the car from resonating off the floor and can produce a lot of dust that others might find sucks, because you will be getting everyone else's cars dirty.:D
Out the sides behind the tires still have to go over the rear end but other than that they are kind of a personal choice over out the back of the car.

I chose something a little different and came out in front of the tires with a slight up angle and cut on the ends to match the body lines. (more resonation at low speeds but fine on the hiway)