Looks like you put a lot of thought into this. I have a few comments to ponder!
First Mach 3 is getting very old and has been unsupported for a very long time now. There are many options these days for a hobby machine. This isn’t a big deal as retro fit at a later date isn’t too difficult. This is just a standard warning for people that might not be familiar with the state of Mach3.
As for the machines table, I’d turn it 90 degrees and flip the linear rails. This would allow for more rigid mounting of your gantry supports and frankly a lot less machining / fabricating. Your table can then be as long as required to get your working area.
Speaking of the gantry if you are going to make those complex supports then you most likely have access to build the gantry beam out of steel sections. Your surface plate gives you a good starting point. As such you might as well make the machine as stiff as the budget allows and the plate can support.
If you do go 8020 I’m not sure filling the cavities will do much for you. The big problem with extrusions is the thin webs where you mount the linear rails. In that regard there is a huge difference in the various extrusions available from manufactures. If you go the extrusion route prefer a heavy series.