Long story sort.  New (used) 3 spd tranny installed to replace damaged orig. one due to clutch not disengaging completely (with young energetic driver not understanding that metal on metal lis not a good thing).  

To eliminate the possibility of the clutch set being bad, new Bprg * Beck on order.  Noticed that clutch fork from donor tranny from 1966 Dart /6 was slightly shorter by 1/4" and was straight.  Mine is 10.8" and slightly curved.  It doesn't look "bent", but it could be after 40 years of service.  

Anyway, I thought the A body fork looked perfectly fine and with the shorter length might help solve the disengagement problem and wanted to use it.  Am I nuts?

Fire away with any suggestions!  and thanks.  

I see no reson why you couldn't do it.  

Through trial and error, I have answered my own question, but perhaps this info can help someone else, so here goes.  The new Borg and Beck clutch set ($132 from Kragen online, price-matched my the local store) solved my disengagement problem and it works fine, as I had hoped.  Also, the 10.5" straight clutch lever (from an A body /6) doesn't quite work on my '66 B body /6.  The bend in the original arm is needed to put the linkage at the correct angle and distance from the clutch fork.  

As it turns out, the correct clutch fork (my orig. one) is also used in V8 applications.   It would have saved me a lot of fiddling if I'd known this beforehand, but at least I only had to remove the tranny twice...

To get the three-on-the-tree linkage to work reasonably well, I have had to tweak the shifter arms by bending them slightly to clear the clutch fork when the pedal is pushed in.  Also, I used a heavier clutch return spring ($3 from Autozone) from the fork running it to the hoop on the engine, to get the pedal to return fully.   The clutch adjustment rod is the 4.8" one.

All this reminds me of my A100 Dodge van that I had back in the late 60's that had even more slop in its column shifter.  I guess that's why people like the 4 on the floor set up, but for this "tight-budget" /6 Belvedere, I'll stick with the old broom in the bathtub effect.  Well, it's not quite that bad, but it's not that great either.  It does encourage slow deliberate shifting, which may or may not go down well with the young male driver of this car.  No one's expecting any miracles here.  

It was fun and fairly simple, aside from having one used tranny go missing which caused a 3 week delay in the project.  UPS delivered the second one without incident.  Total cost was roughly $330 all in.  Hope this is helpful to others.