when I have straighten long peices in house, I have had excellent success, but when you clamp you have to do it 90 deg to the cutting surface. Several vices all trammed in the same planes works well. This way there is no inadvertant straitening done buy the clamps.
Or the best solutions is to send it to a grinder that specializes in dual surface prep. (not neccessarily cheap)
Of course its either time or money.
thanks
Michael T.
"If you don't stand for something, chances are, you'll fall for anything!"