i was thinking a 20mm arch bolted at 90 down the gantry sides, i need to work out where the torsion might be acting... im pretty sure it wont be a staight line top to bottom

All that aluminium profile must have cost a lot
Owch!! £100 meter, it did hurt spending that much but it is easy stuff to work with and its flat enough not to need machining

I intend to look at the servo controller on Wednesday, or this weekend at the latest.
have you got an ESC for that monster motor yet ?