The only time you really 'NEED' a posi is if you are racing, or driving in sloppy weather like snow. I doubt anyone is driving a first gen in snow, but racing is understandable. The OP's 2:56 is a pretty lame gear ratio, and it is only good for top end highway speed. I had a 1980 Firebird with 2:56 and hated that ratio.The 301 had a four barrel Q-jet and it never worked well. I actually back-fitted a two barrel Dual-jet, and it ran much better. My favorite ratio is 3:23, good acceleration, not bad highway rpm's. 3:08 is next best, with an automatic. My buddy had a 1967 Firebird 400, T-400 trans, came with a 3:08 posi. A very nice combination with great top end.

Bottom line is- if you can get a posi rear reasonably cheap, and can handle swapping it in, go for it. But if it is costing a bundle, and a shop must do the work, probably not worth it.