Luckily I do have the data... ARB started measuring Peoria in 1968. The largest book for WLS-A was Apr/May 1971 - in that book, they had a 32.8 share in teens, while WIRL had a 55.2 share.
I suspect that where WLS won the biggest was in areas of the metro where the WIRL night signal was bad. Left with no choice, WLS won.
Dealing with multi-county metros was one of the things that pushed AM stations with bad or very directional signals over the cliff. Or "over the KLIF..."