I'm assuming you have the standard 10 bolt differential, 10 bolts hold on the rear cover. If so it will be an HO52. If you need to, tell them it's for a 1970 model, this rear was used until 72. The seals and gaskets are the same. My local parts store has all the seals and gaskets on the shelf.
Also, you will need a redi-sleeve for the trunnion. It's a thin metal sleeve that goes over the polished shaft, which probably has grooves worn in it from the old seal. Last one I bought was $40, but without it your new seal will leak.
Check your breather cap, driver's side on top of the housing. Probably needs cleaned out, it screws in. If it is plugged, the oil vapor expands when heated and blows out the seals.
