rear axle questions don't know

i have what i think is a 7.25 rear end big bolt pattern but i'm not sure it has a rear cover and i have an 8.75 with small bolt pattern want to use the 7.25 because it is a posi but want keep my small bolt pattern my qusetion is will the axles interchange or are the splines and axle diameter different i'm trying to find out this info before i pull them both apart only to find out that they won't interchange can one of you guys out there give the scoop on it and thanks for any help