What rim size can I run?

No luck at the JY so I ordered 2 rims from summit for the front. I purchased 2x 15x6 rims with 3.75 backspace. I am unsure of the rim size of the back but the current tires on them are 255/60R15 leaving 1/2 from the leaf springs.

I ordered some ties today. BFG Radial T/As
Front tires will be a 215/60R15 and the rear are a little smaller than the ones on it, I chose 245/60R15s Hoping this is a good combo and fitment.