For TRUE duals:
Get a used GT exhaust, and have a shop weld in the two pieces of straight pipe - the cheapest way.
or...
Get an H, X, or Mac's Prochamber, and then slap a GT or V6 cat-back exhaust on - fairly cheap.
or...
Use a GT cat-back system or pick and choose your mufflers and tips, then have a shop weld everything in (minus the cross overs) - fairly expensive.
or..
Replace the whole thing starting at the headers, and work your way back, mid pipe or not, using your mufflers and pipes of choice - most expensive way.