A stock Model A flywheel cannot be used with a stock Model B housing and oil pan. However, a stock Model B flywheel can be used on either a Model A or a Model B.
Pictured above is the stock Model B Ford flywheel at approximately 53 pounds. Note the difference in section and the two machining steps on the OD. The Model B flywheel is smaller to fit inside the Model B flywheel housing and oil pan extension.