Front suspension questions

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