I think this would be a pretty difficult undertaking with questionable benefits.... First off the ZF tranny is actually quite strong in terms of power capacity. I know Jason thrashed his with fuel and boost turned way up for a long time and never was able to break it. (He took it out and then proceeded to break two or three manual transmissions and at least one differential with the same engine that the ZF survived behind!) They also got used in V8 Land Rovers with much more power/torque and almost twice the weight of a diesel Volvo (turning 2 axles and 4 large tires no less), as well as in Jags with large engines, etc. They seem to be quite robust in terms of torque and HP capacity. The main ways they break seem to not be related to abuse from excessive power, but rather to failures relating to the "neutral creep" problem. They can be rebuilt to eliminate the flaws they have and I think the later-production units had those issues fixed from the factory (not sure in the case of your '89). Once their couple of flaws are addressed (or if those flaws don't arise), it seems like there is not much that ever goes wrong with them and they can last a very long time. My daily driver is an '86 745TD with over 295k miles on its original ZF with no problems and I've known of several others with similar mileage.
Second and probably more importantly, the AW71 swap would be pretty tricky.... even if you could succeed in getting a BW55 for a D24 and mating its bellhousing to the AW, which I don't think you could do because IIRC the bellhousing is integral to the BW55 and cannot be removed, you would have fitment problems because the BW55 was only installed to the D24 in the 200 series chassis, which uses a different installation angle than the 700/900 series diesels does.... this would cause the transmission to sit tipped over at an angle when installed in a 700 series. Then you would have the trouble of wiring up the circuit to run the electronic overdrive in the AW trans, swapping in a different shift lever and quadrant to have the OD button and AW shift pattern, shift points for a gasser AW71 might be wrong for a diesel, etc.
No doubt it all could be done with enough time and effort and money, but if you really wanted a different transmission and were willing to commit those kinds of resources to it, I would think that it would be a better payoff to start with some kind of more modern electronic trans (like maybe an AW30-43 out of a 960) and take advantage of benefits offered by electronic mapping of shift points, converter lockup in lower gears, etc?
