67-69 Barracuda FB mini tubed interior question

I'm running 275/40/18 on a 18x9 rim with only a offset shackle kit and rolled fender lip. I have about 3/8" between the back of the tire and the spring pack and about 5/8" between the face of the tire and the body. With the short/stiff sidewall its plenty of clearance
thanks the wheel I am thinking of running on the rear are 17x8 w/ a 4.50 back spacing. I like more outer rim