Thanks a lot guys for the cautionary tips - I definitely don't want to blow this thing up. I'm a master auto tech with only a basic understanding of this type of stuff, but I have learned to proceed with caution over the years! I leaned heavily on this article for the selenium replacement:
http://w3hwj.com/index_files/RBSelenium2.pdf
If you scroll down to the top of the page marked 28 there is an image of a diode setup that I tried to duplicate as closely as possible. To the right of that, under the heading "Replacement Considerations" are the instructions on wiring that I followed, wiring them in series into the chassis. I am hopeful that I didn't goof somewhere, I have re-read it a hundred times!
So, if I want to check for DC voltage when I power it up, I can kind of figure out where to probe with my positive lead, but where do I pick up ground? Being direct current, is it probed at the chassis?