When you get to 45 MPH does it just loose power and quit pulling, or so you just feel it’s revving too high at that speed? If it is loosing power at that speed I suggest you go back to your recent tune-up and reinspect things. As Don mentioned earlier, a few of us have experienced a failure to properly connect the points spring and have experienced points bounce at higher RPM’s. When this happened for me the truck wouldn’t get over 40 and would start to bog down and run terrible. Once I discovered my error, and corrected it, I got all of my power and speed back.