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I've personally never seen a windsor in one but I know i read somewhere that some of the trucks got them. I could be wrong, and i'll keep looking for my source.
I saw a 351 Windsor in a 70 something, 75 or 76 truck at the crusher some time back. My Chiltons covers 76 thru 79 and shows the 351W but it doesn't indicate which trucks or Bronco models got it.
Count your valve cover bolts, 8 = M, 6 = W. There should also be a sticker on the driver side valve cover that gives all the stock tune-up specs that would tell you the engine family and displacement.
Its definitely a 351M unless it was replaced at some point. The good news is that alot can be done with 351M and theres info all over on this forum. Do a search and you'll probably find.
I replaced my 351M with a 400 which is the same as the 351M except internally. This gave me more horses and torque and it dropped right in. Also, Edelbrock makes a nice intake and carb setup with elec. choke you need and emissions correct if you need to pass inspections. Ceramic coated header for the 351M and 400 can be had for about $200 from Jegs (but not at Summit) made by dynomax.
Granted the windsor has many pros too but if you have an M series engine no reason to be sorry. My truck would pull a house off its foundation if I could just get the neighbors to go away for the weekend.