I'm a guy that generally believes 8-3/4's can live if you prep them right, but @ 500 cubes with a 4-speed and B-Body weight, my sense is that you're way past the limit of an 8-3/4. I'm running MW steel maincaps in my 742 8-3/4; along with a Detroit Locker differential, Dr. Diff axles, and a Moser Backbrace. A couple of years in, it seems fine and has held up to my abuse, but that's in an automatic car with 452 cubes, and my 67 probably weighs a bit less than a 69 Dodge. Frankly, I'm surprised Cass sold you the TruTrac (Assuming you talked with him and he knew where it was going??). I would think your only chances for survival involve a Spool, a hefty backbrace, and the steel caps. At that point, the gearset itself becomes the weak link.

Now, all of this is assuming the car hooks at the racetrack, and you're stepping fully into it. on the street with the hardware you've described, you'll likely never see enough traction to break it. If you're going to the track, my advice is: Have a trailer ready for the ride home.