Dear Ward;
I'm not that familar with the flathead 6 and you have done all the tests anyone could do.
Unless the valves are extreamly loose, there okay, as It's not smoking when hot. I am assuming the seals were in there.
That leaves just one thing left.
Somehow the oil rings never seated or were installed wrong, which would not show up on a compression test.
When driving there is a 'mist' of oil mixed during combustion. This would explain why a richer idle improves things too.
Good luck.
