just to follow on I have stuck to the previously tested 2 inches or 50mm that the guys have already used.
have picked a reasonable knob, so not bothered about keeping original.
so to save time I used a bearing puller on the original knob, after removing the top cap and finding the top inner material was soft foam rubber that I cut with a knife for the tool to sit direct on the gear stick.
http://imageshack.com/a/img855/5020/7ggq.jpg
now I do want to keep the lift reverse collar as I don't fancy pulling back for 6th one day and hitting reverse !
so basically so far I have test fitted the new knob and measured where I could cut and glue reverse sleeve.
basically a cut here
http://imageshack.com/a/img703/2323/o41w.jpg
and then a cut at the other end of that section leaving a fairly big flange to glue.
also required this sleeve part inside to be removed
http://imageshack.com/a/img850/5383/ihmj.jpg
to leave a flat bottom, which I then sat a penny washer in with 10mm hole so spring sits snug. this gave enough travel of the sleeve to allow reverse and a good spring down.
but I used a product called milliput (black )to bond parts of sleeve and just checked today and it hadn't bonded at all so just redone with 2 part araldite type glue.
I have glued the faces and then assembled the knob as it will be and then lifted and slipped into reverse to hold the sleeve while it dries, also the inner tube of the knob aligns the parts while it sets, I don't think you would be able to glue off the car as the fit sleeve to knob is very close(may vary knob to knob)
if it sets ok and I use it for a week or so I will strip down again and get some more better pics for on here.