What bellhousing for a 4 speed in a slant 6 Duster

The 64-66 (and maybe 63) was available with a slant and a 4 speed. I think until the 833OD came out, the slant was only available with a 3 speed. It's possible some years of 3 speed bellhousings were drilled for the 4 speed. Take a look at this article:
http://www.slantsix.org/articles/4-speeds/ODA833fourspeed1.htm it's got a lot of good info.