Why did you abandon plans to drill & tap the quill?
Cost..lol the depth into the quill is over 100mm so I needed to buy extra long taps and drill bits. In hind sight it would have been worth it tho as it is definatly the best solution. M8 bolts over 100mm no problem.

Try the cheap option First eh.

Cheers for the stabiliser idea, Not quite what i meant tho. not bothered about rotational postion. I was refering to the the existing head rocking on the post (out of tram(I think..))

In the picture of yours, is that the locking bolts I can see at the back of the head casting?