Limited slip or full spool?

Regarding you original query re spool vs LSD, I agree with the team, go the LSD (Suregrip). The locker (which a spool is) will have no great advantages anywhere except the dragstrip or for stunt driving in a movie.
However, better than both would have to be an Eaton Truetrac centre (available for many rears, I'm not sure about yours). It is tough, reliable, always spins two wheels when the pedal's down, doesn't lock...ever, and won't wear out like your suregrip.
I don't know the full story re your engine/trans/chassis combo, but 3.55s would be great around town, 3.9s even better, 4.10s dynamite within city limits. A holley 650 double pumper will work better than a 750 unless you run low gears, and a loose convertor (>2800rpm stall). Any back-to-back study on the 650 vs the 750 in the real world of the street will convince you......try it, if you can borrow a 650 DP. Dyno shootouts between the two will not see the salient differences......years, and I'm mean YEARS of experience are my source. On a 440, the 750 comes into its own, but we are talking 57ci more, or 15% more displacement. Notably, a 750 flows almost exactly 15% more than a 650 at the same pressure drop.
Powr lok Sure Grip has my vote.

Recently I've been looking at 727 options & posi units for my Newport. It seems there is a lower 1st gear available for the venerable 727 Torqueflite, 2.73:1 vs the stock 2.54:1 stock version. For someone looking to increase the off the line "umph" in a heavy car like a C-Body without sacrificing top end or hwy gas mileage, this (with a looser, 2400-2800 converter) seems the way to go. Those mods should get the back rubber spinning nicely and add some off the line hustle. offers fully built, ready to install units. If your skilled enough to DIY, sells parts.

Eaton posi's are expensive! Depending on your axle, prices start at $600+ and go up from there. But quality isn't cheap, you do get what you pay for. One of these will probably outlive the car.
3:55 is the gear you need, thats what it was designed for, , as an improvement between 3:55 to 3:91 they now do an aftermarket 3:73. Leave the conv alone, a 4000_lb car does not need a loose conv. Daft advice.