Nice to see someone else exploring this. I made the decision as well for my 4.6is, but I have many other projects ahead of this one. I have an e39 6spd trans and pedals, and the hydraulics/shift linkage from the 3.0 as well as a UUC e53 shifter. I've swapped four 540i's so far, and one 525iaT to an M5 touring, so the conversion should be simple. The most difficult part will be measuring the driveshaft and sending it out to be lengthened.

One thing it will certainly need (IMO) is an LSD in the rear. The e30, e36 (M), e28, e34, e23, e24 and possibly others have available 188mm LSD's in a variety of gearing. I feel the 3.64 in the 4.4, and especially the 3.91 in the 4.6 would be too short for the 6spd manual... Considering the 540i/6 came with a 2.93 and the M5 with a 3.15. The e34 535i/5 came with a 3.46 LSD, and the 92-95 525i/5's came with a 3.23 LSD, which would be just a *hair* shorter than the e39 m5's 3.15 ratio. Both should allow the X to get up and move just fine, and would make them quite a bit more economical on the highway.