Yup, I found that youtube link now. I'll try to look into the linkage adjustment again. It is a 47 car and the upper linkage arrangement looks a bit different. It has also had the vacuum assist removed and there are jerry-rigged arms on the tranny itself. But the tranny shifts through all gears smoothly when standing still.
The car belongs to a distant cousin. I am in Minnesota for a few more days, trying to pass myself off as a more of a veteran car expert than I am. I have wrestled with other 3-speeds which lock up, this box feels entirely different. Since the malfunction disappears as soon as speed of the car drops, I still think the problem is inside the tranny.
Which means my cousin can either learn to double clutch or send the tranny back to his rebuilder.

It may not be the easy way, but it is the Cowboy Way - Ranger Doug
Beware of the stories you read or tell; subtly, at night, beneath the waters of consciousness, they are altering your world. - Ben Okri
1953 Chevy 3100
1960 Volvo PV 544
1941 Chevrolet Special Deluxe