Set the ends of front bar so that it is parallel to the ground and measure the sleeve length needed while the car is resting on the ground.
I like the handling with a 5/8" rear bar when combined with a 15/16" front bar.
A bolt-in rear bar is now available from Addco K2-614-0U (special order).
You will need also to purchase a pair of lower mounting plates from WCCC:
https://secure.cougarpartscatalog.com/store/p/13798-1970-Mercury-Cougar-Rear-Sway-Bar-Mounting-Plate-EACH-Repro.html
See the discussion in