I don't know if it's been posted already. Get a long screwdriver. Get a piston at TDC. Doesn't have to be too close. Stick screwdriver down hole so it's contacting the piston. Make a ref mark on the screwdriver with a sharpie. Turn the engine over by hand, and find BDC. Stick screwdriver down hole again and measure difference. Don't scratch the bore. It's very easy to tell difference between 3.3" and 4". I confirmed a 360 based 408 this way.