M8 x 1.25 is a standard size, sounds about right.
The pillar drill trick sounds cool, I'll have a go at that if I cant get the mill, I have a fobco in the garage.
The trick with the swivel is also a good idea, I drive past a landy specialist on the way to work, I wonder if they'll let me have a screwed swivel.
Not sure whether to start the axle or the rusted out rear xmember, probably the latter would be wiser.