Tire Clearance - How much is necesary?

[68 SuperRam]

So need new tires - so looking to change the wheels also.

How much clearance is best to have between the rear tires and the inner wheel well?

Same goes for the rear fender lip?

With wheel off - have about 6 1/2 inches from the wheel mounting surface (face of rotor on rear axle) to inner fender and about 4 1/2 inches from mounting surface to edge of out fender lip.

Currently running 245 50 16 - with a 4in backspace. Looking to go to a 17 - any recommended size?

Rubs slightly on outer fender lip - so probably best to get a bigger backspace.

Thanks

12 bolt axle / disc brakes / DSE 2in drop leaf / original outer fender lip (not - rolled)