Is this enough space on my rear wheels? 18” wheels

Pictures can be pretty deceptive when looking at that amount of gap. It doesn’t look like you’ve got a 1/2” there.

That said, if you do in fact have a 1/2” to the spring, that’s enough clearance. When I was running drum brakes still in the back I only had about 3/8” between my springs and the 275/35/18’s I was running and never had any issues there.

The shorter sidewalls you get with 18” rims don’t move around as much as what you usually get with a 15” rim, but also remember that the springs themselves flex and those leafs slide around on each other and can shift around a little.