Approx half of my lootas\burnas are made from
AOBR boyz. Here are some of my
AOBR burna boyz (haven't painted the Lootas yet)
For Lootas, you will sometimes need to shave down the shoulder joints a little to make the Deffguns fit. Go slow and check frequently on this one, since you can always shave them down a little more, but you can't easily put a little plastic back. For Burnas, make sure you get the backpack in place before you position the arms & skorcha.
In
both cases, you will need to remove, if present, any rows of spike plates that ork boyz will sometimes have running across the shoulders\backs of their necks.
You probably won't need greenstuff or anything, and they are virtually indistringuishable from stock Burnas/Lootas.