Obviously you are thinking too small. I would be looking at a new BMW N74 V12. It's the engine that comes in the 7 series BMW twin turboed and a 7 speed gear box. Awesome torque and power. Might be a bit of a hassle putting it into an R34 though...
You could always just buy an R35 instead of a 34. 3.8 twin turbo has a dickload more torque than a 26 and you would probably spend similar money trying to jam an LS engine in there and make it work with the awd system.
Another idea would be to GTFO...