Hi JunkieHobbo,

Couple of things regards the frame.! It's weight and the ease of build and adjustment.?
I know on my Cad model it looks like the frame is all welded but it's not and actually bolts together quite a bit. Really my models are not fully accurate models regards detail and more for me to visulise so don't pay massive attention to the detail. Thou they are to scale and dimensionally accurate regards materials and compnents which is also why I use them so I can judge component fit etc.

When you build this machine you need to incorparate as much adjustabilty as possible. Thats why the plates on the uprights for the top rail section to sit on.
On the machines I'm building now the main frame(excluding adjustable bed) is actually made up of 5 pieces bolted together (7 if you include top rail sections)
This gives adjustabilty in lots of ways and also helps with moving around.!!!!. . . . . It will be very very heavy so makes life much easier when in pieces.

Regards the plates there's several ways but this is how I do it.!. . . Basicly I machine flat plates with both holes and M12 nuts welded to them which are then weld onto the uprights, taking care to get them square as possible to the boxsection.
!!!!!!! (This next bit will be hard to explain so bear with me.!!!)
The plates are longer than the upright section for the middle uprights so can be bolted either side of uprights into the top rail section which has nuts welded into it. The end upright plates are only longer on one side so bolted on inside from below into top rail then another bolt accessed thru the open ends into plates which have M12 nuts welded to them.
These plates could be drilled and tapped but that would require thick plate so thats why I use welded M12 nuts. The same M12 nuts are welded into the top rail as the 3mm box section doesn't allow enough thread.
It may sound faffy and complicated but in reality it's not and doesn't take long if you have a decent drill press and can weld half decent.

The frame pics below will give a better idea. I'm still building the frames but when get a chance then I'll post a few pics of the actuall steel work, when I'm further along I'll probably start a thread showing it(them) coming along.

Regards the belt drive then there's better ways than I've done it. Mine was rushed and thou it works very well it's not pritty so wouldn't do it that way again or recommend you follow it.
I've done a quick alteration of my frame and mock up to give a rough idea of one way, needs belt tensioner etc but should give you an idea.!! There's other ways if you look around and easily done with motor at same end in line with belt acting as the tensioner incorporation with an idler.

Hope this helps.