Can't garentee it, but I have been told they are totaly diffrent. However what I can say for sure is that an F70 gearbox will fit if you swap the bell housing, and the primary imput shaft over from your old box. They are the only diffrence between the two specs.
The trasfurs are a diffrent mater. The case is identical, but the gearing in an F70 transfure is higher than in an F80. Of cours if your problem is the gearbox, then the transfure can be swaped accross as well.