Since both of these versions were raced in Nascar, the topic has come up over at Randy Ayer's nascar modeling forum. According to a few gentleman over there, the fairlane and custom were two different wheelbases. They are pretty darn knowledgeable over there, and one of the moderators (Tom M) is combining the kits in a build. If I remember right, in one of the threads, there was factory literature to back it up. Regardless, both are great looking cars. I haven't built the revell yet, but it is in my stash (to include the Fireball Robert's version), and I plan to re-do the AMT too (my first was a glue bomb way back in the day).
Cheers,
Greg