I AM enjoying my 4 gears! The ZF is nothing short of revelatory after the BW35. I'll be very interested to see how the LDV box fits into the P5B - please post some photos. For the P6B I decided that the LDV box caused more problems than it solved. It has a mechanical speedo drive (see elsewhere in this thread for a discussion on issues concerning that), but the significant extra length of the tail housing raised issues surrounding the mounts and the propshaft. My particular combination of BMW tail housing and Jag XJ40 output yoke/flange allowed the fabrication of a new cross member that fitted the existing tunnel mounting brackets (trimmed a few mm to clear the ZF), and the propshaft simply bolted back in place. I arrived at that combination through a certain amount of trial and error, but it might be that a Jag tail housing (which I am yet to see in the flesh) is equally suitable, even though it looks longer in the photos I've seen. This, if verified, would make the Jag box the ideal second donor, rather than the BMW.
As for the speedo problem, I can thoroughly recommend the Dakota Digital speedo transducer I've used. It wasn't cheap, but it is the perfect solution, and was simplicity to install and calibrate. It also allows me to change my tyre size with impunity: when I fit my SD1 alloys in the next few weeks I'm planning to equip the rear with a higher profile tyre in order to lift the gearing a little more.
In other news, I've now sorted the Land Rover valve body and reinstalled it, and I have perfect shifting up and down the range, including 4th. The BMW valve body served its purpose, which was to verify that 4th gear was present and correct, and that the problem was in the LR valve body. Otherwise it was mismatched to the LR torque converter and governor, and provided lumpy and poorly spaced shifts. Now the box paces itself beautifully through the gears, and I can revel in the smooth downshift to 2nd it provides as you power through a roundabout or intersection, keeping the V8 perfectly on song. That's something that the 35 would never do. You're moving? OK, you want top gear. You want to accelerate out of a corner? No, you can't have second gear. You can have my patented torque converter rubber band effect...