2.5" exhaust sounds like too much; on wildyoats.com there's an exhaust thread and it says no more than 2.25" to maintain decent backpressure with a header.
Had a similar issue with my 79; put 2.5" on stock exhaust... was a dog after tuning. Found junkyard long tube header, installed with 2.25" pipe and Thrush muffler (also turn-down in front of axle) and the truck is quicker now.
Hell, might even try 2.0" pipe or would that be way too restrictive?
and good luck!
Dan
This supports the lack of backpressure theory....
