Check out this thread viewtopic.php?f=55&t=3707&hilit=MG42 the end result was 2 very nice customised ZIB MG42 replica's using AK gearboxs, one of the MG42s was recently sold on this forum.
249/brick gearbox sits too high. I'm not sure how deep the receiver on a 42 is, and where its bore sits in comparison to the 34, but on the 34 I had to modify a V3 gearbox so it sat low enough into the receiver, while staying within the original constraints of the receiver Jay had milled(dimensionally accurate to a real one). Alternatively you make the trigger group sit a little lower 
V3 is the way to go IMO.
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My armoury;
Tanaka Kar98(1942 woodwork and parts), M1 Garand AEG(1940's woodwork and metalwork), MG34, SVT40, Mosin Nagant M44, MP40, PPSH41, Luger and PPK.
the version three box is cheeply found and spares as well,another to think about would be the version 6 thompson box ,the 249 is a rare box of tricks made for only one manufacturer and if they go belly up then spares will become like rocking hourse manure.also hop units that are compatible are a thing to comsider that favours the more common design of gearbox,personally i prefer the version 6 rather than the more common route of the version 3 but im just contrary.
armoury
m1a1 Thompson,sten mk2,mp40,stg44,sterling,mk2 bren gun,lee Enfield no4 mk1,Mauser Kar98, Walther ppk,smith and Weston m10 and Mauser m712
Give me a big enough hammer and a place to stand and I could fix the world.
i'll kill a man in a fair fight or if i think he's going to start a fair fight or over a woman or.......
a problem shared is a problem halved ,but an advantage shared is no advantage at all
if a job's not worth doing then its certainly not worth doing well
the version three box is cheeply found and spares as well,another to think about would be the version 6 thompson box ,the 249 is a rare box of tricks made for only one manufacturer and if they go belly up then spares will become like rocking hourse manure.also hop units that are compatible are a thing to comsider that favours the more common design of gearbox,personally i prefer the version 6 rather than the more common route of the version 3 but im just contrary.
I would disagree with that slightly as the M249 gearbox can take version 2 gears (the most avaliable gear in the airsoft market) as well as any aeg spring, piston and clylinder head. The M249 Gearbox is also a gearbox which is built for "full auto" and all the proper CA ones are already fetted to a degree. However saying that i have seen the guide and have to agree that the version 3 (ak gearbox) seems the best to use. 
Do version 6's not sit taller from the top of the gearbox to the tip of the motor cage than a V3? Height was my issue with the 34, and alligning the nozzle with the centre line of the barrel.
On that subject, I've seen a 249 gearbox upside down in a 42 receiver, probably to tackle alligning the nozzle and bore issue. No idea if the top cover had room to shut over it or not though. Certainly know there was no room for a 249 box in my 34!
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My armoury;
Tanaka Kar98(1942 woodwork and parts), M1 Garand AEG(1940's woodwork and metalwork), MG34, SVT40, Mosin Nagant M44, MP40, PPSH41, Luger and PPK.
not sure about the relative height of ver 3 and 6 boxes,but the alignment is a worthwile point as the v 3 box has a central nozzle and the ver 6 has it at the top but never forget that cylinder heads can be swaped from one box to another if a spacer is fitted to the front and a tappet plate is cut down,for that matter the cylinder head can even be flipped to move the gearbox further up into the gun,further to that you could cut down a ver 6 cylinder head and fit to a ver 3 box with a cut down tappet plate.i do these type of mods all the time,its fiddly but can be done with a hacksaw ,a dremel and some chemical metal by just about anybody brave enough to try.but i have to agree that aligning a gearbox is easier with a sentral nozzle position,i just prefer working on the ver 6 box is all i supose.the 249 gearbox is reasuringly expensive and harder to find as a complete box.
armoury
m1a1 Thompson,sten mk2,mp40,stg44,sterling,mk2 bren gun,lee Enfield no4 mk1,Mauser Kar98, Walther ppk,smith and Weston m10 and Mauser m712
Give me a big enough hammer and a place to stand and I could fix the world.
i'll kill a man in a fair fight or if i think he's going to start a fair fight or over a woman or.......
a problem shared is a problem halved ,but an advantage shared is no advantage at all
if a job's not worth doing then its certainly not worth doing well
V3 is definitely shallower than a V6.
I dealt with the nozzle alignment issue on mine by fitting a shooter L85 cylinder head, nozzle and tappet plate as the L85 has the nozzle right at the bottom of the cylinder. Brought it to about 1.5mm high of the correct centre line.
I also used a G36 motor cage and trigger.
“I wanted to come to the Volga at a specific location at a specific city. By chance it carries the name of Stalin himself. So don’t think I marched there for this reason – it could carry another name – but because there is a very important goal... this goal I wanted to take – and you know – we are very modest, we have it already."
Adolf Hitler, November 1942
"Comrades, Red Army men, commanders and political workers, men and women guerrillas! It is on your perseverance, staunchness, fighting skill and readiness to discharge your duty to the country that the defeat of the German-fascist army and the liberation of the Soviet land from the Hitlerite invaders depend! We can and must clear the Soviet land of Hitlerite vermin."
Joseph Stalin, November 1942
From the looks of some production pictures, the Viva Mg34 used a Version 6 gearbox.
An M249 gearbox won't fit. Here's a list of gearboxes used in most successful MG42 conversions:
1) Version 3 with AK motor cage
2) Version 3 with g36 motor cage - reportedly a better fit with the grip stick
3) Version 6 with thompson motor cage
4) Version 2 with MP44 motor cage
Here's a link to a US made version using a Version 2.
http://ww2aa.proboards.com/index.cgi?bo ... hread=8308
But in terms of gearbox longevity and stability, i'd still go with version 3.