GSL Rearend 10 bolt?
 

Anyone know if the 84-85 GSL rear is a 10 bolt like the GSLSE? Or is it a 8 bolt?

The diff center sections are a direct swap between 84-85, regardless of whether it is a 12A or 13B car.
In fact, the 79-83 diff centers will bolt into the housing of an 84-85, but the axle splines are very different in size.

So could I use a 79' housing, replace the differential and axles with an 81 GSL, and it would work?

You can direct swap anything between 79-83 with little issue.
You can also direct swap anything between the 84-85 with little issue.
All pumpkins can be swapped into housings from any year.The diff. drive flange size changed in 83,so driveline fitting must be adressed.

When you start mixing the two styles of rearend,you have to adapt to deal with the changes in axle shafts and axle bearings that happened in 84.The pumpkins all have the same bolt pattern,and you only have to change the side gears in the diff,to change to the bigger/smaller axles.But in order for the axles to be fitted from one style housing to another,you have to change the wheel bearings on the axles too.

I tried 3 times to reply to this earlier, only to be met with the stupid "Server is too busy" message.....

Steve has it pretty much nailed.
The axle shafts themselves are the same from 79-83, regardless of drum/disc brakes. The axle housings are different, though. Drum brakes have 4 axle retention bolts, while the disc only uses 3. This complicates trying to swap disc brakes onto a drum housing. It can be done, though. RESpeed offers plates to add rear disc to the RX3/4 rearends, which would allow using a (79-83) drum housing and disc brakes. Handy, if you happen to have a bent disc housing that has everything else in good shape.

AWSOME! This is my current situation and now knowing this makes me happy. Thank you both Rogue and Steve