3:90 Gear Ratio.
You have a couple options;
a. put in a 3:55 from Patrick's with a new bearing kit willl cost around $550 for parts.
b. put in a late 70's thru mid 80's blazer rear end, advantage here is you keep the 6 lug pattern, get bigger rear drums and the width is a perfect fit, so all you have to do is weld on new spring perches. From what I understand you can get 3:08 or 3:42 gears
c. Late 70's camaro rear end, here you can get drums, 5 lug pattern but you can also get a 3:09 or 3:42 gearing. If my memory serves me correctly some come with discs and some with drums.
When I had my 55.1 I was going to go with the blazer rear end, pretty common and I could get a good one with 3:42 gears from a boneyard for $75.00, rebuild the drums and I would have been good to go.
Hope this helps,
Chris