hey i got a 9" in my 73 mach1 350 gear not posi want to make posi what is cheapest way

 

BUY a Powertrax, This is a no slip unit that makes a non posi to a TRUE posi ,,,both wheels lock posi.
When you turn the posi kicks out and you will hear a clicking noise, but when straight and when burning rubber its true posi.
I have one in my 8 inch and love it, They say it makes a none posi rear stronger that a factory posi unit.
Its small and very easy to install.