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Da Boyz
By James, pics from GW
In general, 40k models are slightly larger than GM models. Also, GM models are mounted on smaller bases. This means that mobs using the normal GM transport rules (as opposed to Houze Roolz: Tranzport) will be at a disadvantage. I recommend mounting them on smaller plasticard bases or something to try and help with this.
Other than that, I don't know what I can suggest.
Anyway, most GorkaMorka models are easily substituted for 40k ork models. Here's some examples:

Nob
There's a selection of very cool 40k nobs (a plastic one comes in the 40k Ork Boys box, metal ones in a Nobs box), and the more crazy ones of you could even experiment with GW's huge ork Warboss figures. These guys all look dead ard as nobs.
Another idea, suggested by GW's Adrian Wood, is to use an Ogryn's body and add an ork Nob or Warboss' head.
Nob
Spannerz
The 40k Mekboy / Spanner Boy and GM Spanner are actually identical models. So there's no problem in just using the 40k model.
Spanner
Slaverz
Once again, the 40k Runtherder and the GM Slaver is the same model. The Runtherder comes with a bunch of grots as well, which is useful. If only it didn't have a 40k base...
Slaver
Boyz
GM plastic ork boys are a bit weedy. Much better to use the new 40k models (metal and plastic). The one displayed is a metal stikkbomber. Generally just buying a 40k Ork Boys box would provide you with plenty of ork boys though, as well as a Nob. And they all come with choices of sluggas and choppas or shootas.
Boy
Yoofz
There are no Ork Yoofs in the 40k range. This is a problem.
I recommend either using GM models (metal yoofs or the plastic boys) or just use 40k Ork Boy models, and paint them with lighter coloured skin. Orks get darker skin with age, so this is perfect!
No Picture Available
Grotz
GorkaMorka and 40k models are identical, except for the bases.
Grot