Sorta kinda half-assed fixed my cut problem. I figured that since the feed rod is not guiding anything, there is no reason it needs to be snuggly secure at each end, just so long as it transmits torque from the keyway. Not a long-term solution, but something to get me running for the time being. It 'works' in that I got no ridges and even got about half a barrel turned cylindrical (was tapered, which makes squaring it up a very tedious incremental process, especially when using a follow rest) with a decent finish.
The only problem was when I then went to work on truing up the backplate to be mated to my 4-jaw chuck. Crank the carriage all the way up to the head stock and the cross slide all the way out past the ways to work on it, and the ridges come right back. Seems that even loose in the journals, the shaft will bind to one side on the drive keyway, just not as forcefully as when the snug journals were camming the shaft around. Quick solution was to remove the feed rod, use the compound for the edge cuts and chamfers, then use the cross slide to manually face the plate. Seems to have worked nicely, a smooth, consistent 'brushed' gray surface is the result, which I assume is about as good as you can get with cast iron. Scribed a circle for the new bolt pattern (either I am getting tired, or the cross slide dial markings do not correspond to what the lathe claims; I couldn't hit the 4" diameter target until I rigged up a dial indicator and used it to set the cutter position).
I think having to crank the cutter out past the ways (to where it tries to tip down over the front of the machine) to cut the outside diameter of the plate was about 90% of the cause for trouble. It seemed pretty consistent when doing <2" diameter stuff, as far as trying to cut rings into the profile.
Since the feed rod is hexagonal, I have been 'truing' it in my 3-jaw, which means there is probably still about half the original runout no matter what I do (which may explain some of the lingering issues). If the new feed rod is also wonky, whenever it shows up, I'll have to buy a hex collet and borrow a buddy's collet chuck to get the journals dead nuts true, and make some bushings to fill the gap created. I'm really, really, really hoping that a square feed rod won't show these grooves, because that would mean the adjacent gear train for sure is the culprit.
TCB