i have an 86 trans am rearend.
stock 55-59 truck is 62'' wide, 82-92 trans am is 62'' wide also.
check out these charts on chevytrucks.org
scroll down to see the rearend info.
lots of good reading on that page/site.
the 86 trans am also came with disc brakes and a 3.27 gear.
i averaged 20.1 mpg on the hotrod power tour.
most any rearend you choose will need all the 'stuff' cleaned off and spring perches welded on.