Light throttle 2-3 upshift at 40 mph is way too late, stock is around 15 mph. Stock carb or aftermarket? If aftermarket does it have the correct Mopar throttle lever extension?

If so, try disconnecting the linkage and let the trans lever come full forward....drive it easy and see if it downshifts coming to a stop. If it doesn't it's either a governor or shift valve problem.