That what I like to see a man getting on with it and not talking about it. .

Couple of suggestions.! Put some triangle plates on those upright legs to support them.!! . . . I know your thinking is he on drugs this things built like brick shit house but When machines cutting you'll be surprised just how much those legs resonate and this will transfer thru the gantry and down to the finished cut quality.

I'd also do something on the inside to help keep the frame square, either with similar triangulation braces or plate with holes in each corner then threaded rod between to pull square.

Because you have welded the top rail box section it means your linear rail mountings are going to have to be cock on parallel because you have no adjustment. It also means your top rail surface or plane is fixed so again if you have any discrepancy between each side it will be difficult to get out.
If you bolted it to plates then you have a lot more freedom to shim and adjust into parallel and same plane. There are ways around this like Epoxying the top rails so they level to each other but this is more involved. Depends what you plan to do with the machine but building in adjustment helps greatly with setting up.