OK, well we made a major breakthrough over the weekend .. 
   
I never knew that if I modified the airbox (so that it would fit under the hood) that the slides would not work properly .. someone even told me thaat they WOULD work, but at a higher RPM .. anyway this was the problem .. this was why the engine would not go over 5,000 rpm .. obviously the vaccum was not high enough so the slides were not rising so the engine was running too rich/restricted, etc ..
   
Well, we put the airbox back together properly (albeit .. with duct tape) and took it for a test run and straight away we were getting the full rev range .. and boy has that made a difference .. even the 'grabbing' clutch I can probably live with now as the power comes on smooth and theres no 'bucking' (is that the word?)