My understanding was the 489 case had cone style suregrip, the 742 case had the rebuildable( and tougher) clutch style suregrip. But either are fine on a street strip car
Either of those two posi types are WAY smoother than a Detroit locker, which ratchets.
I had a Detroit locker in a Duster for a while, never, ever again. Get out of the gas at speed under hard excelleration on the street( 9 sec car) and the car wanted to dart around like crazy. Darn right creepy.
Zero such issues with suregrip or spool, both very predictable, work fine.
On a mostly street car, the suregrip is hard to beat