I have an 87' bird with the 2.8l and an automatic. As far as i know it has a non posi 3.73 in it. My question is what is my best option for swaping in a posi unit. Or do i need to get a rear with posi already installed. Ive heard that the 88 and older vehicles with posi where having issues with the posi going out early. Another question is what spline are the axles. I do have drums as of now and an entire new rear would be nice for the diskbrakes. Motor should be producing around 400-450 at the crank and going through a 700R4 when im done, and maybe a hint of nitrous.

What about stiffening up the 10 bolt, as in i know a better rear cover is a good way to go with oil cooling and making it stronger, but how about welding more support to it?

 

I thought 2.8 engines had high gears. If you can find a local pick your parts junkyard around your area, I would suggest just getting an 88-92 3rd gen with a limited slip/posi. These will have the 28 spline vice the 26 spline that are currently in your car. Here is the codes for the posi unit and codes for the gears.
G80- posi/limited slip
GS4 3.70
GT4 3.73
GT5 4.10
GT7 3.33
GT8 4.10
GU1 2.41
GU2 2.73
GU4 3.08
GU5 3.23
GU6 3.42
GV2 5.83
GV4 3.36
GV8 2.72
GW4 3.31
GW6 3.27
GW9 2.93
As far as making the rear stronger, you can get an aftermarket rear cover from EBay or summit racing.