I too am researching McClain/McLain/McLane/McLean in Nelson & Marion counties, KY., and they are also one of my brickwalls.
All the name variations make it extremely difficult to sort through the family lines; I was working w/a double 3rd cousin on Mcleans and their ties to White, Bray, Harp(e)& Martin families in same area. Unfortunately, he passed in June of 2010 and I've not progressed very far w/o him.
I have a William Patterson McClain, s/o John B. McClain & Mary McClain. Mary is d/o Arthur McClain. Is this your William?
My gg-grandmother is Margaret Harp m. Henry Bray. Margaret's parents are Richard Harp & Elizabeth McClain. I do not have parents as yet for Elizabeth.
Good luck! I'm planning a 2nd trip to Nelson county this summer, hopefully I'll find some new info.