Recommended posi rear end?

You should look for a 8 1\4 rear end they have the large bolt pattern wheel bolts, the better brakes that are readily available at any parts store and usually have 321 gears. Moparaction.com should have an article that outlines the procedure for making the 81\4 as strong as the 8 3\4. The only real advantage of the 8 3\4 is the drop out pumpkin. Stay away from the 355-1 and lower gears on a driver they are too much.
Mopar trucks have posi diffs and the diff oil for lockers from mopar says "posi" on the container with a mopar part number. Good luck and keep asking questions to get different opinions.