1960 Model B GMC V6 401 NAPCO Conversion. Little has changed, but we do use her regularly NA 

A 6-71 ought to fit in there. IT does without the napco conversion anyway... I wonder if the bellhousing for the 401 is different or if its a standard SAE bellhousing ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-u-kTtQkXXgg.html

The case has no tag that I could find. I thought it was a Rockwell T226. I can tell you low is 2.9-1.

As I understand, these drivetrains were always made by "others" Timken, Rockwell and more. You can have good luck with parts in hand if you are trying to renew. If you go to the parts counter and give them year make and model, you are doomed....