8.75 rear in 71 swinger slant six car?

It is true. The towing package included the 8 3/4" axle due to durability. I have seen many ho-hum cars come with the 8 3/4" axle. Keep in mind that by the end of the 1972 model year, they may have been stuffing them into anything just to use up the last of the inventory.
I have seen these axles in several non performance cars. Last October I traded for an 8 3/4" axle that came from a 4 door 71 Valiant.