Rear axle/ gear help 1980 mirada

I have never changed gearing before , have done a ton of motor work but the rear scares me.lol
I have the 8 1/4 rear and was wondering if anyone would know of one that would be a direct bolt in but with better gearing or would this be something a newbie could attempt on his own?
Mt apologies for being green but after the shop quoted 1500 if I buy the parts, I am looking at alternatives.
Author: cowtipper