2 1/2 inch exhaust on a 318...

How much power does the engine make. Be conservitive rather than probable best day and tuned for power. You gave a hundered HP difference in power. And thats a whole lot at the level your describing.
Street bound cars can live with a compromise of alot of variables on the build from front bumper to rear tail pipe. Race cars, dedicated ones, are set up to extract everylast bit of potential there could be in the engine and chassis.

On mufflers, the chambered units I like are from Hooker. There Aero Chamber. Less powerful cars can use most any other chambered muffler with good results.

For the straight through units, Hooker (Again) and Dyno max have excellent choices. The list really goes on & on. It's a personal prefrence.

IMo, if your looking to max out the combo above 470 HP, an honest 470, 3 inch pipes.