68 Barracuda. How Big Can I Go? Rim and Tire?

Before I jumped off & bought wheels & tires, I'd jack the car up, take the current wheels & tires off, put it on jack stands under the rear axle & lower control arms . From there, I'd start measuring for back spacing, front spacing & over all width. Most of the tire manufacturers have the specs for their tires so you can make an intelligent decision. Just my .02.