Nick
Engine torque shouldn't have a permanent effect if the steel was the right spec.
MGBV8's with parabolic springs are very prone to this, and there has been a lot of discussion over the years. A BV8 will visibly squat over the rear offside wheel under hard acceleration. In fact a guy who drags a BV8 at Santa Pod gets the nearside front wheel off the ground due to the twist.
The other force at play is that the axle tries to twist back if there is no anti tramp bar and forces the leaf spring into an S shape. This also has the effect of reducing the camber and lowering the back end, which can become permanent.
Good luck with the dealer - I imagine they will say it's within parameters.
