Replacing the rear axle seal on an 8" is a piece of cake. remove wheel. remove drum. remove 4 9/16-head 3/8 fine thread nuts holding axle retaining flange. pull axle, replace seal. re-assemble. $5 part. 30-minute job, tops. You'd spend twice as much time and thre times the $$$ to put an 8.8" in its place.
No, 8" and 8.8" are NOT anything alike! The 8" is like the 9"s baby brother. Both have center sections that load into the rear end housing from the front. The 8.8" is more like a Chivvy 10-bolt. Loads from the rear.