Hi,

Connecting the gantry at the bottom will give little benefits but, by moving the legs far apart, the base will be a lot weaker.

Why would you put the X rails "hanging"? Only for dust/swarf protection? The side plates extension for guide blocks will deflect a lot and put pressure on the rails/blocks trying to twist them. I don't think it's worth it.

The gantry side plates are beefy but only when looking from one direction. If you cut out one third longitudinally and bolt it perpendicular you will have more rigidity in the Y direction without any loss or weight gain. And you can connect to your t-slot profile overhang.