Is 1/2" leaf spring relocation worth the trouble?

They are Mustang wheels and the next available offset on the 10 's is +45.

That brings them in too far...add a spacer?

I think you'd be better off with the 65-67 B rear and the 18x10 +24's.

With the 18x10, +45's you'd be looking at using the 68-70 B body rear, and then you'd probably need a 10mm spacer which is quite a bit of spacer. It would take longer wheel studs to get it done. Unless you ran disk brakes too, in which case you'd still probably need a 5mm spacer.