Like I said, hand held, isolated scopes are nice things in some applications.
All you really need to keep burned into your consciousness is that the little croc clip on the scope probe is connected to the earth and neutral wires in the plug.
If you can get your head around that then everything else becomes obvious.
The normal procedure is to plug the DUT (in your case your spot welder) into an isolation transformer. This lets it float with respect to ground and means that you have to touch two points on it to die instead of one.
Catch is, an isolation transformer that can handle the power your welder should draw are big and expensive. (maybe you could get two more MOT's and wire them back to back??)
So what you would probably do is plug the scope into an isolation transformer and float it WRT ground. NB, this is QUITE A LOT MORE DANGEROUS because the case of the scope including the front panel (do the knobs on the front have metal grub screws in them that your fingers may touch when you adjust the settings?) will sit at whatever voltage you connect the earth clip to....
Having said that, there's no reason not to do this, just plan ahead and write a short note bequeathing all your tools to me in the event of your death.
I have a bit of a process I like to follow when I work on high voltage stuff I'm not confident with. I take a digital multimeter set to AC volts and hold one lead. Then I poke the other lead onto whatever bit of metal I'm thinking about touching. If I see more than 20VAC on the meter then I know I've just avoided electrocution. It's not a fool proof process, but I'm still here..
Oh, and RCD's are a MUST HAVE for this kind of work..