You could tap a single phase of the three, depending on how the three phase outlet is wired (wye or delta). Check with an electrican.
Tapping one phase only might damage the generator. I would ask on a generator forum.
I'm not familiar with the M7 but we have some WW II era army and navy generators in our collection and the three phase ones are all "wye with the neutral not brought out"
As for using the big radial arm saw from a carpenter's trailer to cut slab wood, I'd go with a chainsaw. It would be easy to make a trestle to hold the slabs and an out feed table for the cutoffs to drop on. With a helper to clear the cutoffs away you would go a whole lot faster than a radial arm saw.
If a piece of the slab wood moves even a tiny bit against the fence of a radial the blade will bind and you are very likely to have what approximates an explosion when the slab gets grabbed by the blade and flung. A big saw blade and direct drive motor have a scary amount of energy stored and a piece of wood flung out by the blade binding can go right through a wall. I've seen it happen. Given the tapered and slightly rounded edges of the slabs I don't see how you are going to avoid the issue over any length of time.
Also, be aware that the saw blade will keep spinning for as much a 25-30 seconds after you shut the power off due to the large rotating mass. A lot of people have been seriously hurt or killed working around them while making repetitive cuts.
BTW: Black and Decker owned, for a time, the division that made DeWalt industrial tools and any DeWalt industrial tool parts house would be the best source for parts for a B&D industrial radial arm saw. They were DeWalts with a B&D nameplate.
Lance