Rearend w/posi upgrade - need advice

I am wanting to put a "better" rearend in my factory original 69 Mach 1. Ratio is high - 3.0, I think - so I am wanting something like 3.50 or 3.70 with posi. Car will be used for occasional cruising, not raced, but I want it "snappy".

First, I don't know if it has the 28 or 31 spline axles. Also, should I buy a complete 3rd member and swap out the old one or buy the necessary parts to build it from scratch?

Either way, I would also want advice on which brands or companies to go with. I see a lot of stuff offered on Ebay, but don't want to go the wrong way.

I am also about to have the engine built. I will go with a fairly stock setup - only adding a cam, carb, intake combination to increase stock hp to something around 320-340 hp (that's another post for another day!).

Ok, that's cool, but what's the cost/benefit ratio by going to a 31 spline? I always assumed that I could make a strong enough street machine with the 28's.

In other words, if I am going to the trouble and expense of changing the rearend, is it a no-brainer to go ahead and upgrade the axles as well or would that just be a luxury considering my mildly-wild intentions for the car?