Trying to figure out what I have

If it originally was a slant six car like you said it was by the 5th digit of VIN being the letter C chances are it probably came with a 7-1/4 axle from the factory....I highly doubt it was optioned up to the 8-1/4 rear....especially with 3.21 gears....Chances are that was put in when the 360 was put in...