No.....I don't like any of them, except the last one.....the main reason is the 2 base tubes are not forming a rigid box frame with bolting and that will lead to misalignment in the 2 Y axis linear rails........they have to be dead in line.....not even a smidgeon out to one another across their entire length.
Bolting them together will give you no end of trouble trying to get them to be both level along their length.
This is one reason why in all build the Y axis rail seats are machined together to make them "perfectly" flat and level to each other.
You would not have any reference to test if the rails are level to each other.
My suggestion is, referring to the last pic........lay the base rectangular tubes down on their bigger side (without the mounting pads) and weld the two Y axis tubes onto them......the rest of the design is as per the last pic.
I would also strongly suggest welding the 2 uprights to the bottom of the crossbeam, not bolting them.....this will give you just one complete assembly to bolt to the base frame.
Once the base frame is complete the 2 linear rail seats and the side supports pads are all that needs to be machined.....that is, the main machining apart from the mountings for the stepper motor supports etc and a few tapped holes.
Machining the base in one hit gives you accuracy.....the X axis assembly with the side supports can be "fitted" to the base by mating them to the base.....the X axis rail seats can be machined true to one another in 1 plane and square to the bottom of the side supports.
You'll be able to get the two assemblies close to one another with the welding and final fitting, to give you 2 axis that are more or less square in 3 planes and just need final adjusting to make them good.
Referring to the last pic, I would make the uprights from the same rectangular tube section as the base and cut them away for the width of the X axis tube to make them continue up the back side of the X axis crossbeam where they are welded back and bottom....this will give you a very broad based pair of uprights all in one piece.
I think with a frame design like this you could get 200mm under the X axis for a 6040 build.
You only have to get the two base/side support interfaces mated to one another to have a machine that is almost done.
Ian. .