I designed the table in two halves so that if we (my son and I) decide that we want to do something different such as a narrow layout, but longer then we have two tables 900x1200mm end to end (900x2400mm in total) and either way we can add more modules as time progresses. And it also make it easier to dismantle should the need arise - easier to put two 4'x3' tops in a ute than a 4'x6'.
With regards to using 70x35mm, this is what Bunnings has readily available. I have tried other dimensions, but the lengths were restrictive. At 70x35mm the minimum length was 2400mm, others were 3600mm or 4800mm and ended up costing about the same +/- a couple of dollars. I have also spoken with other hardwares, but the delivery fee and surcharges were far less with Bunnings.
The cross supports I can compromise with, but I would rather keep the center ones in place to further prevent sag, but from experience it will also provide a bit more rigidity to the sides preventing any torquing of the timbers even with the top on.
I am being a bit over cautious with this build as I have taken short cuts or have underestimated a need only to have it bite me in the butt big time - I blew up a video card over the XMAS/New Year break because I underestimated my needs.
I really am appreciating the feedback too. Just have to convince the wife that the layout belongs in the disused formal area and not is a dark, damp, dingy garage.