8.75 Sure Grip spider gears break

Have you thought about how your unit works?
At very light throttle the crosspins sit in the neutral zone and it acts sorta like an open diff.with very little preload on the clutches.
Under power the torque differential drives the crosspins up the ramps and locks it all up like a spool.
Under heavy braking, or compression braking, the torque differential drives the crosspins up the backside ramps, and again locks it all up.
If you happen to be in a turn, something has to give; either the unloaded tire has to slip, or the clutches have to. If the tire slips, then the clutch pack will stay locked up.
I doubt the clutches will slip except during shifting.
So then if you are down-shifting in a turn....... the clutches will go from full reverse-lock , to full slipping, to full forward lock. Now as they do this, the spiders climb down and up on the crosspins going from maximum backlash,to minimum backlash, and back to maximum. So they are being jack-hammered mercilessly, hundreds of times during the race.
Same thing happens when upshifting, but now you are backshifting to a lower position on the power curve, so it might be a lil easier on the spiders.
That 4-pinion Sure-Grip is actually overkill for your application, and as mentioned, not a good match.

But if you swap to a regular Cone-Type Mopar Sure-Grip, you need to customize it to your useage. Which means minimizing the spider walk, up and down the single cross-pin, and then setting the clutch preload. You do this with the oil you plan on using, with the SG additive already premixed into it, and all the guts prelubed. Then I recommend 120Ftlbs turning torque as a starting point .
I do this by using an old cut off axle clamped in a vice, splined end up, and slide the case down over it, assemble the guts, then drop another stub axle into the top and tighten it all up. On the stub-axles, I have welded some large nuts. Now I put the torque-wrench on the top nut and commence turning it. I'm not interested in the break-away torque, only in how much it takes to slowly make it slip. Adjustment is by thickness changes in the side-gear washers, in conjunction with spring preload. If the guts are used, you will have to die-grind the steps off the old patterns, so at their new backlash setting, they will all play nice together.
120ftlbs is just right for launching my 68 barracuda in a straight line, with both 295/50-15s blazing. But around turns on a clean dry surface, the inside wheel still slips at normal driving gas pedal application. And I can adjust the slippage with power application, making huge 360* powerslides so much fun, right on the torque peak.
I don't know the correct setting for your car; you may end up changing it several times. Alternatively; someone here on FABO has done the testing already, and may spill the beans. I can tell you this; 100ftlbs was not enough for me, and 140 was too much. Third time, 120, was a charm, for me.
Whatever you decide
Happy HotRodding