Hi Mick,
Well I just checked my AutoCAD drawing and from the bottom of the chassis floor rails to the centre of the top wishbone bracket is 255mm, not sure what book measurement is as I haven't used that in a long time (don't even know where it is..lol).
I'm guessing moving the wishbones back 20mm might be alright the only problem would perhaps be the position of the wheel arch in relation to other components i.e rear tub and radius on side panel.
Not much help I know but I guess somebody will tell me I'm wrong somewhere.
