Fuel injection all the way, that's my personal bias. I've done probably 4 960's when I worked for Ross and they are the most difficult but if you go ahead and buy the Volvo factory wiring book for the 960. Like 50 bucks, it'll make life so nice, Volvo made a very complete wiring book for all of the models. You can find all of the signals you need and really all of them are nicely packaged in the large plug right in the engine compartment fuse box.

Doing a later explorer or mountaineer EEC V injection system is definately more challenging and really not neccessary unless you need to have OBD 2 for some reason. I'd be inlcined to find a nice donor 5.0 thunderbird that you can pick up for 100 bucks and use its injection system, convert to mass air and be on you way.