posi, positraction, safe-T-trac, twin grip, anti-spin, sure-grip, etc, etc. are all limited slip differentials, just with different marketing names. http://en.wikipedia.org/wiki/Limited-slip_differential Now there are several different types of limited slip, the S-spring kind you have pictured there, or there are Eaton, Auburn, Torsen, etc.
Try to talk him down closer to $300... offer him $300 and if he says no wait a week or two and raise your offer a bit. I suggest swapping the entire rear end into the car.
If you do swap the entire rear, do yourself a favor, do what I did. Pull the axles out of both rears. Then unbolt the backing plates and tie them up, and drop out the rear, bolt in the replacement, bolt up the backing plates, and install the axles. No messing with brake lines, or emergency brake cables.
For an 8.5" rear end I wouldn't pay over $300 for an S-spring type no matter what, or $300for an Eaton type of unknown condition. I 'might' pay $400 for an Eaton type if it was rebuilt, and I don't mean the seller says it was rebuilt, but it actually was rebuilt, aka trust the seller or see reciepts for new friction plates, etc. Brand-spanking new 8.5" Eaton carrier, $477. 30-Splines http://www.quickperformance.com/YDG...for-GM-85-86-with-30-spline-axles_p_5357.html 28-Splines http://www.quickperformance.com/YDG...on-for-GM-85-with-28-spline-axles_p_5356.html *Edit* Is that $500 for the entire axle assembly? That might be worth it in that case if you just want to swap the axle assembly as a whole and want that gear ratio. It's another $100 for new gears for the above carriers and then likely pay someone to install the gears and carrier in your car which would bring the cost up another $100 or so bucks for a ball park $700 for new gears, limited slip and install.
let me preface this by stating that I'm not racing my cars at a track. I run a 3.33 gear set up in my GS455 and find it ideal around town and its not too bad on the highway. I have a 3.42 in my GN which is ideal for that cars drivetrain in town and on the highway. I have a 72 suncoupe with a 430 and 3.08 gears that Im currently building and I believe will be just a little less seat of the pants in town but should still have plenty of get up and go and should be better on the highway than the 455 with 3.33 gears. Needless to say, they are all positracs/limited slip rears. Open rear Peg legs, one lungs ecetera pretty much render a BBB useless except for tire shredding. Cant get 4000 pounds moving in one direction too quickly on one leg.
