i went from a 42 rear sprocket to a 38 tonight, I buzzed up the road for 3 gears, seems faster everywhere , this weekend I might see what the Gps reads

The 38 tooth will allow the engine to operate at or closer to the power peak for a longer period of time than the 42 tooth sprocket.  Make sure you have high octane fuel, the right heat range spark plug, and the right main jet for the atmospheric conditions!

Which transmission are you running.  87 or the 88 to 90 model transmission?