I took Ed's suggestion to try electrical tape around the anode lead, and it worked - the arcing was eliminated. After addressing the 5V supply issue with a new filter capacitor, I was able to see the beam with the beam finder - so the scope is back alive, if not yet fully functional.
Reed, you may or may not remember, but I met you a few months ago when I stopped by your place to pick up a couple of 475s and a Pyramid Generator I'd purchased on ebay. I've learned a whole lot since then! Not that I don't still have a whole lot more to learn. And it was the parts junker you threw in from which I got the replacement Multiplier module.
I really appreciate all the responses. When I made my first post, I was ready to throw in the towel after having spent so much time trying to figure out why the HV wouldn't start up, and then gutting the scope and the parts unit to get to the Multiplier modules only to have the arcing issue happen after reassembly. You guys really helped. Thanks!