Thanks Andy, will call KJN tomorrow, they don't list the price for 160x80 on their site.

Last time I used a 6"x2"x0.25" aluminium box section, it worked ok but wasn't particularly flat or straight. I'm hoping I can use the slotted profile gantry without having to shim the rail. Milling the gantry is a pain as my mill only has 700mm of travel so would need to move an re-reference and would loose accuracy.