4 pinions are stronger than 3, 5 are stronger than 4!
The bolt-in sprag is a good idea, as is a billet drum (so I've heard).

The Gear Vendors unit doesn't decrease the load on the transmission, so transmission strengthening is still a good thing.

You need to find pictures of the two different Sure Grip units and look at what you have. If it's a 489 case, it should have the cone type, if a 742 case it should be the clutch type and if it's a 741 it could be either. And, of course, anything could have been swapped into any case over the last 40 years!

Can you swap your 3.23s for 4.11s? Sure, this is a job for a rear end specialist, but it's done all the time. The 3.91 gearset seems to be the most popular.

R.