[Crazy Dazz]

Further to what Raptor has said, I too am not following.

I had a couple of wagons, and yes driving on gravel roads was an adventure. The rear effectively "skips" across the bumps, and it becomes directionally unstable. And yes, it would be worse in yours with the tougher springs.

I'll be honest, I cannot fathom how this is ripping your rear tyres.
Yes, stiffer springs means more impact is absorbed by the tyres, but that shouldn't result in what you are describing.

The bad news is that with that HD suspension and LPG, I honestly do suspect that your tyre are under-inflated.
The only other thing I can think of, is that somehow this rebuilt suspension is out of true and the car is crabbing?
(And I'm assuming that your bearings and axles are ok?)

Stiffening up the front is not IMHO a good idea given your situation. It may make the car better balanced to match the back and the front, but over rough roads it will make it worse.
As others have said, I can only suggest returning the rear closer to stock, and fitting good shocks all round.
I'll be honest, I have never stripped down a leaf rear. But I've done coils and found it easier than many suggest.
If you hunt around, and you're prepared to ull them off yourself, you can probably find somebody parting out a wreck for cheap. Not sure which models were the same, but can't imagine there was much difference in the leaves.