Ok I'm planning on getting new rear gears and a posi unit this winter, right now I have 2.73s non-posi, so I was thinking about getting a cheap posi off of ebay and either 3.23s or 3.42s off of a performance parts website, so I have two questions...

Are 3.42s too much for a barely modded 305?

And

About how much do you think I will get charged to get a shop to install them?