check some of the rear end sites and match up the gasket with your cover..
if its all stock? you probly have a 7.5" with 2.40 to 2.73 gears,,with a 2 series carrier..
for 3.73s youll need the thick gears for a 2 series carrier or
a 3 series carrier with normal 373 gears..
and the swap does take some special tools and has to be setup right for it to last..:texas:
try a search,youll get lots of hits..
To expand on what JJ is saying- ring gears are mounted to a carrier. for the lower ratios ( up to 3.08 ), a series 2 carrier was used. For 3.23 and above, a series 3 carrier was used. You can use gears in a series 2 carrier, like 3.23, 3.42 or 3.73- but only if they are special "Thick" gears. If you want a ratio from 3.23 up, and don't want to buy the thick gears, then you will need to swap the carrier. You most likely have a series 2 carrier.
Elky's came stock with 7.5 rear ends. If you have a 3 speed trans ( most likely), then 3.42 is about as high as you want to go. Any steeper ( like 3.73) and your highway RPM's will be well over 3000.
Easiest swap is to find a rear from a 78-87 MonteCarlo SS. They came with 3.42 from the factory up until 86, then switched to 3.73. They would be a direct bolt in. Much easier and cheaper than trying to change the gear ratios of your stock rear.
Here's a pic for reference of what the 8.5 rear looks like. The 2 "ears" on the bottom of the housing are larger and more squared off on a 8.5" rear out of a g-body. But as mentioned above, the 8.5" rear isn't stock equipment on our cars, I added this out of a Grand National. I hope this helps you out.
You gotta' give us at least a little time. Does your drive shaft simply spin with no movement of the vehicle? Could this be a transmission problem? "Not going into gear" usually means a transmission problem, not something in the rear end.