El Beast - Initial design phase, comments and critique welcomed!

[Voicecoil]

Interesting. Just to be clear the part of the gantry that is actually doing the work is 65 x 200 mm made up of a heavy 40mm extrusion and a 25mm tooling plate, not just the 25mm tooling plate on it's own. The span is 620mm... it also has 6mm plate top and bottom which will be screwed down to the extrusion and the tooling plate. Given the relatively short span I don't think it should have much flex?

Yes, I got that, the extrusion will help, but I suspect it would need to be bolted to the plate at a lot of points to get it to work " as one": if they're not joined seriously rigidly, you won't get the full benefit of the extra thickness as they can slide over each other. If you want to stick with your current arrangement, I suspect that splitting your plate into 2 and having some in front and some behind the extrusion would also be of benefit.
OK your gantry isn't overly long, but if you're machining metal you need all the stiffness you can get. And as the rest of your machine is so beautifully made it would be a shame to compromise this surely?