How hard would it be to swap the hubs from a 91 legacy onto a 88 gl? Any input would be greatly appreciated. If this is or already had been discussed somewhere can someone please put a link in this thread.

You can't, they're entirely different.

The fronts MAYBE if you also swapped the strut, drilled the tophat mount, found a balljoint that fit the control arm, and made a custom axle...

The rear, no chance. 

The hubs by themselves are not swapable. The fronts can be done by installing the legacy struts with loyale tophats and reaming the control arm to take the legacy ball joint. you then have to use a impreza 2 wheel drive axle to tie everything together. the rear has been done by swapping the complete suspension but that makes the tread width to wide.

 

I'm sure someone who has done it will chime in shortly with more information.