Sure, no problem. I’ll give you several of the key links, and each is self-explanatory for it’s purpose original size ref, upsizing, offset/clearance etc. you’ll find most of them cross-linked inside of each page but just sharing them separated here for you:
1968 Mercury Cougar - Wheel & Tire Sizes, PCD, Offset and Rims specs | Wheel-Size.com
Rim & Tire Size Calculator. Custom Offsets - Wheel-Size.com
And for reference, The Legendary Wheel Co. (owned by Holley now since they bought out Scott Drake) wheels offered are beautiful finishes & all taken from original 1960s, early ‘70s OEM designs and are built to fit on our cars without issue: Legendary Wheels (Official Site) - Classic Muscle Car Wheels & More | Holley