a 1973 XJ-6 is a Series 2 XJ-6. The Series 1 XJ-6 was made from 1968 to 1972 though there was some crossover-leftover into 1973. It depends on the car's Serial Number.
I'll post an XJ Production Chart later-gater. (assuming this site allows picture posting).

 

Picture posting is allowed no problem.

 

depending on your location, I can refer you to some salvage yards where I get parts for myself. But Rotors can be had new from AutoZone, Car Quest, O'Reilly's, you name it, they're cheap, why buy used? Refuse to pay a lot just because it's a Jaguar. There's caliper repair/rebuild kits too. All rubbers can be had in Red Polyurethane if you want, not expensive. I'm assuming you bought the entire rear "Cage", right?

 

For the record, a 73 is a Series 1 car. Has the same interrior, and body style as the rest of the Series 1 cars. The main difference is the front bumper was changed to include the rubber overeriders. Every other part is compatable.

Your gearing ratio is probably a 3:54, identified by a tag on the differential (46/13)

I recently helped someone acquire a Jaguar rear end to put into a Mustang. His wesite is at Dazecars.com. He is in the process of rebuilding it now, and has a new posting on JagLovers.org with a place to get all the rebuild parts for less than $400. Check it out......

Good luck
David