MyBB 1.6 enhancements

[ZiNgA BuRgA]

You're pretty much attempting surgery to create some hybrid 1.6/1.8 here, which may work, but probably won't unless you're sure about what you're doing.
You could try to diff some files between the two versions then port code changes across.  Alternatively, you can try searching through MyBB's Git history and apply commits onto your copy of 1.6.  But you may actually find it easier to just port your code to 1.8, rather than port 1.8 to your copy of 1.6.