what is a fair amount to pay for a '87 GTA rear end with 156k on it, no breaks otherwise complete 3.73 28 spline posi, it does come with the GTA sway bar this would be a direct bolt in replacement for my 90 drums just get the disk prop. valve right?

 

I assume the rear is a 9-bolt (which would have 3.70's). If you replace the brakes with the original 87 Delco-Moraine calipers, you won't find a combination that will fit the thread type on your 90 (though an adjustable is an option). Best bet might be to upgrade to, say, PBR's, and purchase the correct 1LE valve from Ed Miller. You'll also need to swap the drive and/or driven gears to make the speedo read correctly.

You also might consider a search on 9-bolt limited slips and worn out parts.

JamesC