Thanks Bill and Peter...

Bill - the Relmac is a 4.5", 16" between centres, the SuperRelm is the same but 20". The headstock is integral to the bed, so we are talking 55kg+ of cast iron to move around! I don't have another headstock of any usefulness available; well I do have the one off the Lorch, but that has no useful taper to attach a boring tool.

Peter... that has given me an idea...

On the back of the headstock is the banjo plus a stud for an intermediate gear. If I was to make something that would bolt to the banjo plus this stud that just cleared the back of the spindle and held a bearing I could pass a 3/4" bar through the spindle with the far end located on the tailstock centre and into the bearing. I could then position the bearing to locate the shaft centrally in the spindle referencing a centre in the tailstock. I can then remove the bar, remove the spindle and replace the bar knowing it was now aligned with the tailstock and therefore with the ways. If the bearing allows the bar to move parallel to the ways then I can use the tailstock ram to push the boring bar into the headstock. All I need is some way to drive the bar round as it moves and to keep it located in the tailstock.. I am wondering about mounting a direct drive motor clamped onto the tailstock ram. I need about 600rpm and if I'm doing a .5mm deep cut at 0.1mm/rev (about 60mm/min, or 30sec to go through the bore), I'll need 80W of cutting power...

Thats a lot less construction effort probably..