Rear Suspension

[chevy55]

I have finally decided what I am going to to for my suspension on my 55 2nd series truck, but I need some help.

I decided on doing four wheel disc brake and I am going to bag it.

My question is does anybody else have problems with the rear fenders rubbing the tires? Whenever I would get on it or hit a big bump the truck would sink down and the fenders would scrape one of the rear tires. What is the best way to solve this problem?
One idea that was suggested to me is to shorten the rear end, the differential in it needs to be rebuilt.

Which leads me to my next question, what are the possibilities for the stock rear ends? I want to change the gear ratio to something like 3.43 with a posi.

Any suggestions would be helpful