Elevator rigging problem

[B2C2]

My 1969 T337D is in for annual. as part of that I am having them redo the rigging to deal with a problem where about 10-15 degrees of right yolk rotation was required for level flight. Flaps and ailerons are now re-rigged and correct, but my A&P is having trouble getting adequate cable tension in the elevator cables. the turnbuckles on the cables in the strut are bottomed out and the tension is 10 lbs, where manual says 20 +20-0. Has anyone seen this before and can suggest a solution? Adjustments in the flight controls area are completed and the yolks are parallel and he seems to think this area is correct. Further adjustment there will cause the yolks to not be parallel. Right now we are thinking about replacing the two cables that lead from the strut back to the elevator suspecting cable stretch, but my mechanics are suspicious that this should be required. I should mention there was a rear prop strike incident some time ago before I owned the plane that required the replacement of the boom that carries the cables, and although it is not mentioned in the log book I'm wondering if the cables may have been replaced at that time with cables of incorrect length.