Thanks guys.
As it turns out, I tried something a bit different and it seems to be working:
The 0-80 bolt I got from bob2 is 3/32" in length over all. Since there was no "play" if I tightened it all the way, I thought maybe a slightly longer bolt would do the trick. I shortened a different 0-80 down to 1/4". That extra 1/16" made all the difference; I can now tighten the bolt fully (so it stays tight) yet there is a little "play" for the eccentric rod to move freely.
The chassis is now (more or less) reassembled and is turning at a nice 3V; will kick in at 2V off my Heathkit power pack (which is a whole other story). Still need to put it on a track, but I'm feeling pretty good about this project right now...
Question: the tender has no rivet detail; is as smooth as can be. Can that be right?
Mark in Oregon