Gearbox suggestion?

Hello,

 Trying to finish my big rebuilding of G for offroad. Putted OM605 2.5TD, maded mechanical with OM602 mechanical diesel pump. For oil radiator parts from OM606.
Some pic 1 2

Now the biggest chalange is to find manual gearbox. There is one possiblity to find from sprinter 110 spacer to put old gearbox. But because I put slowest axles from army G old slow gearbox didn`t want. Now I installed all with gearbox from spirnter 2.2cdi. Had few problems with installing it, but the biggest problem - clutch slips, even without boggers, with original tyres. Ok, I didn`t put new clutch kit etc, but I got it in very good shape. This week I think to get new one and to test it. But thinking in future give my pump to myna disel to get more horse power and thinking, is there better manual gearbox to suit my needs. Or only options to get automatic gearbox if I have so match power. Any suggestion? Maybe there is other manual gearbox from MB what works good with lot of power?
Maybe somebody has suggestion what to do with clutch?