I think you should save your money and find a used turbo kit.
I mean... You aren't going to see any noticable difference(except for noise) in going to a massive exhaust system.
If you want to improve it, just take whatever you have and throw on a glasspack muffler. Make sure the end of the exhaust exits /outside/ the truck(not a dump underneath), and call it good. Save your money for the turbo.
Remember, you are totally limited by air /into/ the engine. I tried for the longest time to get better performance from my truck at the time, and, really... not a whole lot of difference no matter what I did.
Put a turbo on it? Definite improvement. Crank up the fuel to match? Massive improvement. Install a big enough injection pump to max out the turbo? Significant improvement.
And, once you have the turbo, near as I can tell, a single 3" will do just fine.
Heck, my 93 is currently making 250 on the dyno - a NA makes about 125. What do I have? A stock NA motor, factory turbo kit(with restrictive downpipe), straight 2.5" stock pipe and a nice 110CC IP. It /rocks/.
(I had the same pump on an 88 with Banks turbo and 3" exhaust with a glasspack muffler. I *also* got approximately 250 on the dyno. Stock/kit turbos just aren't big enough for more than that)
Sure, you can spend as much money as you want on exhaust components, cold air intakes etc... But for more power, you need more air and fuel. Nothing gets you air like a turbo. Simple as that.
And, if you watch the wrecking yards, Craigslist or car-part.com, you should be able to find a used kit for $500 ish... and I've seen exhaust kits in that range.