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
