I have a 86 f350. I'm trying to figure out if I have a 351m or a 400. Can anyone help

 

In an 86?
The valve cover EPA dating down below on the label shows the engine (or at least the valve covers) to be for 1979 compliance. 351/400 covers were interchangeable as well..

Per the decal data...

Engine 'Family' shows 351M/400
If you look at the 'Engine Displacement' noted below the Family,, it states 6.6 which is 400, and 400CID (400 Cubic Inch Diameter)
351M would be the 5.8 or as they list it as 5.8M

 

Ok thank u I tried to read forms and figure it out but the was too much arguing back and forth