2 inch Dual exhuast should flow well enough to support your engine. Usally its the pipes that go over the rear axel that produce the highest restrictions in stock systems. A larger pipe in the restricted areas is best if its a straight shot 2inch is fine.
Mandral bent pipe will help make the 2 inch work better. Also 2 1/2 right after the collector will help with more flow. But under 5000 rpm 2inch should provide plenty of punch. Larger full legth headers and 2 1/2 will be worth about 15-25 hp. So its not alot to give up even if its not flowing its best.
A badly setup system could cost you a few ponies more than it should. So be sure to get a nice welded up system. Alot of muffler shops will weld something up they bend for little moeny on older trucks. Some will also try to make a lot of money. Best to shop around or buy a flowmaster or other universal kit.