Lots of people on here have done a spring over axle (SOA)... However I would suggest purchasing a new axle with brakes. Anything over 3000lbs requires brakes legally in a lot of states, and it's really worth it if you ask me. The additional stopping distance with just 3500lbs pushing you will surprise you, especially in an emergency situation.
Like the poster said above, just make sure you'll have the clearances for the bigger tires on your wheel wells and adding a set of shocks will also help greatly.
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