Headers from a 302 will be shorter than headers you need for your 351w, so your stock y pipe will not reach properly. Heads from a 302 will work on a 351w only if you expand the head bolts from 7/16'' to 1/2", and I would recommend a professional do it to make sure the holes stay square. There are not many EFI intakes for the 351w, and 302 intakes will not be wide enough to reach the heads. A cam upgrade will be your best bet to liven up your truck and your OBD II system will handle the changes.
Even more simple is to change your gear ratio, go to a higher set to either 3.73 or 4.11 and that will set your power band lower in the RPM range. Anything you want in performance, you will lose in mileage. Push rod engines are not efficient.