Marvel Chronology Project

Billy the Kid's chronology currently consists just of his appearances in Rawhide Kid vol. 4. That's pretty weak. Here are some more appearances:

All True Crime #42c
Black Rider #11e
Cowboy Action #11f
Gunsmoke Western #71a
Kid Colt Outlaw #14d, 17b
Lawbreakers Always Lose #2e
True Western #1a
Two Gun Kid #80a
Western Winners #6c
Wild Western #18d, 30d

I forgot to add that it looks like there may be a Billy the Kid appearance in Two-Gun Western Novels Magazine vol. 2 #3, at least judging by the text on the cover (I don't have a copy of my own to check).