Machining reference surfaces on a steel frame.

I am building a dual gantry ultrasonic scanning tank system. It
consists of high grade thk rails, a steel frame over a tank of water,
linear servo motors from Copley and linear encoders from renishaw.

Well actually is consists of a hole in a project bank balance at the
moment as most of the bits are on order. But while I wait for the
rails and motors I need to finalize the frame design and start
getting it built.

The frame is going to have 4 legs and then a square outline at the
top allowing full access to the tank from the top and the sides. The
reason for my post is that the renishaw encoders should have a 0.8mm
ride height +/-0.1mm. Over the length of travel on the long axis
this may be difficult to maintain although and changes in flatness
should affect the rail as well as the encoder keeping the ride height
fairly true in theory.

That aside it would still be really good to get the tops of the frame
where the rails will be mounted as flat as possible. The longest
area is about 1135mm long and the frame will be around 800mm wide.
This is pushing it even for the big Boko milling machine we have.
And, the surface finsh of the milling has been a problem in the past
and I am not sure how accurate it will be over such areas.

Blanchard grinding has been mentioned in the past, this seems to be
the manufacturers name and refers to grinding with a vertical spindle
(horizontal wheel) and a rotary table. I have seen it refered to as
something else in the UK as well as rotary grinding and plough
grinding.

Basically has anyone any ideas on this? What sort of tolerances can
Blanchard grinding give and are the cutting forces similar to normal
grinding i.e. will it be possible to machine the top supported only
by the 4 legs?

Another thought I had was to find a huge surface plate and then use
moglice or similar to produce a bearing mounting surface.????

Cheers,

Graham