Sorry, I couldn't resist.

Actually the diff cover can help you discover which rear end you have. Someone is bound to post a site where you can match your cover to a picture and then figure out which rear end you have.

Short of that, remove the diff cover and measure the distance across the ring gear.

Still too difficult? Build a motor with 550 hp, install a 4000 stall converter, mount some serious rubber on the rear, and try 'er out. If she holds, you have a 8.5 inch. If not, then you start looking for one.!!