Dave from RND Engineering posted in another thread that you can change cams and alter timing with engine in place. Don't think you can change the followers though (although PetrolHeadPete somehow managed a full head off rebuild with the engine in (big job!)
OK,looks like the main question is the removal of the rocker shafts. I can't see why they would be a problem, presuming that the grub screws can be reached at the back of the head, but hopefully one of you guys who have been there before can confirm. Craig