Easiest way to tell a 250 from a 200(or 170 for that matter) is the starter is below the oil pan rail (since it uses the same bellhousing pattern as the small block V8) or that the water pump uses 4 bolts versus the 3 for the 200 and others.... The 250 is also over an inch taller than the others...
As for a 170 vs. a 200, the 170 has 3 freeze plugs on the pass side of the block, and the 200 has 5... excepting the early 200's which only have 4 bolt mains like the 170... but I think those are pretty rare by now...
Best cars to find a 250 in I would say is the Maverick/Comets, and Granada/Monarchs... as far as junkyardin' goes...
