Rear end advice for racing with a 4 speed

Your clutch is what twisted the axles. The more torque capacity a clutch has, the faster it pulls the engine down when you dump the clutch, which results in a sharper impact sent to the drivetrain. Most install an aftermarket clutch and think nothing of putting a 650tq capable clutch behind a 450tq engine, not realizing that excess clutch torque capacity is counter-productive and leads to bog/spin/broken parts problems. A proper clutch for the OP's 340 only needs about 2500lbs of clamp pressure on an organic disc to hold the engine after the shift into 4th gear. Aftermarket PP typically has quite a bit more, simple solution is to shim the aftermarket pressure plate until it barely holds after a WOT shift into high gear.

A pretty common misconception is that you only want the clutch to slip during launch, truth is there is more to clutch tuning than just dialing in the launch. For proof just look at the data logs from any fast adjustable clutch car, it's pretty common to see half a second to a full second of clutch slip after the shift into high gear. They could easily adjust their clutch for less slip after the shifts, but data tells them the car is quicker and more consistent with the slip.

Even after the PP clamp pressure is optimized for the application, you still won't have enough clutch slip for a proper radial launch. Simple way to get what you need during launch is to add a ClutchTamer. A 'tamer momentarily reduces clutch clamp pressure, which in-turn allows the clutch to slip longer than it otherwise would in 1st gear. That gives the car more time to gain ground speed before the clutch locks up, which helps keep the engine operating up where it makes more power.

Grant