The strength of the rear is the same, regardless of the engine originally installed. They all used the 8.2" 10-bolt. You probably have 3.23 or 3.55 gears in the one you have.
You can buy a posi unit for the rear you have, it just needs to match the gears you have. There are aftermarket units that are plenty beefy, like the newly released Eaton unit, and the stock 2- and 4-pinion units are more than adequate for a 350.
