To break it down:
Bore spacing is the same and cylinder head bolts are in the same location, so heads are interchangeable with modification to cooling passages, and bellhousing holes are the same, but that's where the similarities end. Clevelands have "canted valve" heads (the valves are twisted and tilted) whereas Windsors are inline like a Ford FE, small chev, holden or chrysler. Cleveland heads are visably wider which gives the impression the Clevo is bigger. Cleveland has no cooling passages in the intake manifold so the thermostat housing is in the block rather than the manifold (this is the easiest way to tell them apart). The crank centreline to cylinder deck distance is 9.2 inches for Cleveland, 9.5 inches for 351 Windsor and 8.2 inches for 302 Windsor (including late model 5.0 litre). No other parts interchange. I mentioned the heads b/c that's where the majority of the power is made and where a bigger difference is besides internally in the crank area.