I can give you my experience re modifying the gear stick, and hope the info helps.
I had to adapt my shifter because its position was uncomfortable for my arm. I bought the aluminium accessory that lenghtens the stick and positions it higher and backwards. The new bit was sort of S shaped. Well it wasn't long before the aluminium bit started rotating on the original steel stick, with the 'female' aluminium thread becoming more and more disfigured with tightening and use over a month before finally snapping off.
I ended up getting a steel rod of same diameter, bending it into the S shape and making a thread on the ends with a tap-and-dye (metal place did this). Then welded 2 lots of 2 nuts together (could have bought longer nuts but all closed late Sunday!). One set of 2nuts went over the original stick (to below the thread) and the other set went over the new stick, far enough back to expose the thread.
The new stick was positioned on the old and the 'new stick nut' was screwed down to eventually half cover the original stick thread (where the gear knob sat).
The 'original stick nut' was screwed upwards to meet the other and was tightened against it.
My gear knob was then screwed on.
Have used this setup for over 18 months now and works perfectly. If I could post a pic I would (working on it). Can show you at next meet or message me to view in Belconnen area.
