[67ss]

I don't buy the pinion angle post. 2-2.5 degrees down should be sufficient in relation to the trans or you can measure off the damper. If the damper is 3 up then the pinion should be 5 down for a difference of 2. When the pinion climbs the ring gear the pinion angle will go up and your goal is to be parallel with the trans yoke or 0 degrees pinion angle going down track. Pinion angle on these cars has no bearing on traction but more on driveline efficiency. I used to run the hop stops and stock type lowers and never had my pinion angle at 0 or up nor did I see the pinion angle go negative with rear end separation.

I ran that same tire but not the ST. I ran the L8 compound tire. The M5 is a harder compound but if your issues are in the heat that shouldn't be an issue and maybe needed with a transbrake launch to keep from shearing the rubber. Are these mounted on at least 8" rims?

You could kill some of the hit with a gear change to say a 4.33. I know my 396 gave me fits with a 5.13 so I stepped back to a 4.56 and tamed the car down.

Some rear mounted ballast isn't a bad idea as well.

Work on the front end and some of the other incidentals and hopefully you will see some progress.