Free Flowing Exhaust

[ExtremeJDK]

So I was wondering about an exhaust issue. I recently got a stock '88 5.0 5-Speed. No real modifications to motor or gears yet, but a bit of weight reduction (AC, rear seat, misc.) A friend has offered me a complete exhaust system with full-length/long tube headers, pre-cat dumps, cats and mufflers, but my question comes before all that. What kind of HP increase and or time drop in 1/4 mile would one see coming from this stock exhaust stock powered pony to the long tube headers with exhaust dumps directly after them. So basically a motor with long tubes on them and nothing else. Of course thats for the strip only, but I don't need every single pony possible while on road. Thanks for the patience with the simple questions!