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Here's the easy way to check the stroke without removing the head. Get a straight 12 inch piece of coat hanger wire. Get a magic marker. Remove the number 1 and number 4 spark plugs. Turn the engine over until the timing mark is at TDC. Put the wire in the number 1 spark plug hole with wire resting on the top of the piston. Make a mark on the wire even with the lip on the valve cover. Now move the wire over to the number 4 cylinder and mark the wire again. Measure the distance between the two marks.
For those that don't know: 3.98 is the stroke for the 410 and 428 engines; 3.78 is the stroke for the 390, 406 and 427 engines and 3.5 is the stroke for the 352 and 360 engines.
Thanks to John Wilkerson for giving me the idea for the stroke checking.