Someone tell me I ordered the right thing please

Okay, time for a little help. This really applies to aluminum rims, but can be interpreted for steel.
An 8" rim is measured from inside bead to inside bead. So, ad in the half inch thick beads, and its really nine inches wide. But backspace is measured from the inside edgr to the mounting face, so a 8" aluminum rim with 4.5 bs, has the mount face exactly in the middle of the rim, zero offset.
So, a 15x7 alum. rim is eight inches, and 4 inch backspace is ALSO exactly in the middle of the rim.
So. Both will have the tire in exactly the same place, but the rim, AND the tire sidewall will be 1/2" closer to the spring and fender. The eight incher just needs more tire to match its width, so automatically eats up a lot more clearance.
A 15x7 with 4 1/4 backspace will be a 1/4" closer to the spring, and 3/4" farther from the fender than your 15x8 4 1/2 bs would have been.