Start narrowing it down if you can't access behind the starter. Visual looking at it from the pictures you provide I am saying right off it isn't a 351. The valve covers would look to be further under the front shock mount brackets. But that could just be the angle.
But for the sake of reference, a 351c has 8 bolts in each valve cover, as opposed to the 6 in a 351w. Also, the radiator hose connects to the intake manifold on the windsor, and directly to the block on the 351c. Once you get past that, determining if it is a 351w, vs 302 or less is a matter of looking at the retaining bolt on the distributor to the block and how it lines up, height-wise, with the bottom of the intake manifold. The bottom of the nut will be about 3/8 of an inch (I may be wrong here) from the bottom of the intake manifold on the 302, 289. There will be 1 inch (this one I know) of space on the 351. Even easier, the intake manifold on the 351w is 11 inches wide as opposed to 9 1/2. Hopefully a few of these help you narrow things down. Also, if our eyes are fooling us, and this is a 351w, you can tell if it is an early 351 w because the intake manifold is being held on by 16 bolts as opposed to 12. Good luck with your search for answers!