You can tell the difference for sure between the 251 motor and the 265 by measuring the stroke. Remove the plug (not spark plug, there is a small hole placed right over the piston for timing purposes) over the #6 cylinder, and place a piece of welding rod in there as you turn the engine over by hand.
If the stroke is 4.75", it is a 265. If the stroke is 4.5" it is a 251.
