A welded diff in the snow is a terrible idea. it sends the back of the car sideways and you end up fishtailing until you stop gaining speed.
I would recommend a 5 speed transmission swap but i do not know if that is possible at this point. Also get some larger tires. not sure what selection you have where your at.
You also might want to strengthen the underside of the car, or make skid plates, or something of the sort
You shouldn't hurt the tranny in low gear as long as you dont excede 35 mph/55kph. Those trannys have a thing for blowing up 3rd gear and reverse though, in their old age
Also, a brush/deer guard, offroad lights and high output alternator might be a good upgrade
For tire size, 215-75-R15 is a good tall, skinny tire for snow use and turn radius in the fender well, though some cutting and bashing is required
What are you going to do to keep the engine warm? I assume you will be in some freakin cold places, dont want the motor to freeze.