Have you considered unscrewing the tensioners, setting to high, just as if it were a standard fitting, tighten the nut to lock and pushing up manually and repositioning the screws? Might be worth unscrewing one side to see if you have enough distance from the original holes for a secure fitting. You could fill the original screw holes with a glued wooden plug if it’s feasable. If you are lucky the tensioner plate might mask the original screw holes. If not perhaps a leather patch or ally plate to increase the size a bit.
Seems the only thing controlling the position is the tapes on the roof bars that need to be tight. Having said all that I have never done it myself but I think as it’s already overtight before fitting to the windscreen the position of the roof bars is the controlling factor ?
