Thank you all.
Frazer, the Harold hall website is where I got the idea of machining these from.
Sandy, firstly I will face, drill then bore two posts for a M8 clearance hole, while each is in the chuck ill use a revolving center in the tailstock to support it while the OD will be turned, making sure the bore and OD are true to each other. This will of course mean making sure there is no taper in the lathe, and that it's levelled correctly. Though if I can setup to within 1thou over 6" then I'll be happy. It is a cantilever bed after all though that mill does help with twist, last time I checked it, (after 3months idle it was writhing 1thou over 4" and this is over winter too lots of movement.
The posts will then be parted off and faced the other side, this side isn't a reference and is just somewhere for a washer and nut, the tee bolts will be coach bolts filed flat on the sides and a little to the dome top to fit in the Tee slots.
These can then be clamped to the cross slide and indicated with a DTI against a steel ruler held against them to get close, fine adjustment will be with a gentle hammer tap and the DTI measuring the high spots against the rounds.
This setup should mean that I have two truely vertical posts 90° to the bed bolted to the cross slide that are parralel with each other and then I can setup the angle plate.
This will be dialed in as best you can on a unmachined surface, and this cut will set the datum for all other sides, what you say about the 3.5" available cross slide travel is a good point and one which hadn't occurded to me actually. I suppose the only option would be to move and indicate the freshly machined face. The other option been to do the machining for the larger one with it mounted to the faceplate to skim it. I know I can get to about 0.00025" tolerance across the face. The setup been the same as previously described only turned 90°.
I was wanting to machine it all at one setting using the mill. But I will have a think on that one, faceplate would get upto 7" of travel from the 3.5" on the cross slide, so more than suitable, though I only have a DTI that measures in increments of 1thou, and is also a plunger type so could introduce error of reading from its angle of dangle to the surface.
The machinists clamps came about as I will need them for the Clayton eventually but will be useful for a multitude of sins.