New build help needed

[JAZZCNC]

Few questions and I think I am good to go.

Steel tubing for the whole base 60 x 60 x 5 ? Do you agree on that?

Well kind of but only has a minimum. I'd be more inclined to go with 80x80x5 on such large machine but 60x60 will work fine.

Length of the bed 2600 + 100 short overhang + 300 long overhang = 3000 total would that be o.k? Do the overhangs need to be braced?

What are the spacing of the cross members and the top rail supports in your drawings? No dimensions so I can't calculate.

Dimensions will depend on your needs and gantry etc. I only drew this frame to show you and save explaining via text.
That said the dimensions I used where based on previous machines I've designed and are very close to those you mention.
From memory the short overhang was 150mm and the large overhang is the same length has the gantry width. Bracing on large overhang depends on gantry width but if more than 300mm then yes brace it.

Here's a 4x4 machine built using similair frame design but with adjustable height bed. Frame is 60x60x5 box.

Has anyone joined hiwin rails lengthwise let's say 1500 + 1500 to achieve 3000mm length. Is it doable for the home builder? ordering from BST automation 3000mm rails will make shipping cost way up.

Yes it's commonly done on really long machines but does need careful fitting. Thou If possible I'd avoid doing it.