seeking wisdom

The only Mopar gearboxes that were 18 spline were some of the 440's and the hemi cars. None of them to my knowledge were original to A body gearboxes

If you have an A-833 box with "something like" 10 splines that is less than 18, what you have is a "made to fit GM" OD box. Some of the OD boxes were made WITH GM input shafts and GM bolt pattern cases. So far's I've read, these are nearly useless to a Mopar owner, other than perhaps some of the internal parts.