here is a picture of my truck totaly supported by the tb's

Huh? Your truck is always supported by the torsion bars. There is nothing else. Shocks do not support a vehicle.

As far as "how much", you will run out of suspension travel before you run out of "twist" in the bars. The bump and droop stops will only allow things to go so far. Unless you crank them such that the UCA is on the droop stop, there is no way to "preload" them. Without the stops coming into play, they will always come to equilibrium, hence, no "preload".

Michael