Carry the flywheel/flexplate with you to an industrial fasterner company and see if a 1/2" fits the hole in the flywheel. It should be a close fit on it but will seem much tighter in the crank flange. So a 1/2" might not be too tight, they usually have several thousandths of interference fit which can sometimes make it seem like they are too big if your trying to push them in by hand.
