I got a set of 38x12.50 TSLs on 15x8 6 lug steel wheels. Bout 60% tread, a little cracking on some of them (older tires) but they all hold air and wheel fine.
They are not grooved at all, and even just cutting the wear bars out would really bring them back to life.
I was just gonna let the wheels go with the tires... I guess I'd take $450 if I keep the wheels. and yeah, there is lot of difference between a 38x12.50 and a 42x15