Robert,
Congrats on the build, you've done a great job so far!
I'd strongly recommend not using grain timber for any structural parts as it's almost impossible to seal it enough to stop warping and expansion/shrinkage.
If you absolutely have to use timber go for MDF or good quality plywood.
But as you doing such a fantastic job so far, it would be a real shame to use inferior parts, even if it does build the cost up :-)
Keep up the good work, and post lots of photos, apart from myself there would be many people very interested in following your build!
One thing you may find with the feet so high on the corners is that at higher speeds you may get a bit of a rocking action affecting joints and precision ultimately. Maybe add another two to the middle to more distribute the footprint of the machine? But I'd certainly lower them myself anyway to the lowest practical level to lower the centre of gravity of it.
One thing that is always good, is make the base as heavy as possible, this acts as a sound dampener, and limits flexing problems. I'd add some supports to not only strengthen it as John Z suggested, but also add mass to the lower centre of gravity.
cheers,
Ian
It's rumoured that everytime someone buys a TB6560 based board, an engineer cries!