I will attempt to answer your questions to the best of my knowledge, but keep in mind that I have owned neither a 740 TD nor a 960, so I have no direct experience with either.

I believe the front crossmember for the diesel is specific to that engine, so you would have to change it to put the TD in the 91 740. As far as I know, the mounting holes on the unibody for the crossmember are identical on all 7 and 9 series cars, so that part of the swap should be doable without too much trouble.

960s came with several different axle ratios, depending on year and transmission type, some of which would be suitable for use with the diesel. (somewhere in the FAQ section is a complete list by year and model) They also came with three different kinds of rear suspension. Sedans all had IRS, with coil springs until 94, and with a transverse leaf spring from 95. Wagons had a live axle, at least until 94, perhaps later. I have seen them with leaf spring IRS too, but I don't think any ever came with coil spring IRS. Most if not all coil spring IRS cars had Nivomat shocks, which are very expensive to replace, so a live axle or leaf spring car would be a better choice, as they often had conventional shocks, and live axle cars can easily be converted with parts from an earlier 7/940. The leaf spring cars, 95 and later, use wheels with a different offset than the earlier ones. You can put 850/V70 wheels on these cars, and 15" or larger 740/760/940 wheels will fit the earlier models with the standard RWD offset.

OBDII was introduced on 96 models, from all manufacturers, so you should be able to find a car earlier than that with no difficulty. Some states will give you a hard time if the model year of the engine is earlier than that of the car it's in. On the other hand, diesels generally don't have to go through emission testing, so you might be OK, it all depends on how things are done in your state.

I don't know of any 960s for sale at the moment, but I have seen several of them, with engine problems, go for a few hundred bucks each at the local wholesale auction. A couple of them were very clean and nice. For some reason, sedans seem to be a lot easier to find than wagons.

There used to be a guy on here by the name of Mapleleafer, who was thinking about building a 960 TD. It is an appealing concept---all the comfort, convenience and safety features of the later car, and 35 MPG. I think it can be done, but it would require a lot of thought beforehand, and a lot of work to actually put it together.