can someone tell me a easy, positive way to tell a ford 351W from a 351M ? The vehicle is a 1980 ford f-150 4x4 Trailer Special (dual front shocks) I heard somewhere the # of valve cover bolts are different bettween the two engines

Yes what you heard is true. I have a 1980 F-150 4x4 with a modified. Here's your answer: 351M has only 7 bolt's holding valve cover Plus 351M spark-plugs insert stright in/downwards as well as the valve covers are shorter and rounded. 351W has 8 bolt's and the spark-plugs go in at a slant towards the firewall.