slant 6 question

peanut heads started in 1975 still solid lifter, til 1980. and yeah, there are performance cams available all the way thru the run of /6s... many have taken to having their original cams sent out and reground, which is another option.
998/999 versions came out in late 70s advantage was wider gear spread in 1st and 2nd vs regular 904. There never was an OD automatic made to directly bolt to a slant block. people have used the old '60s smallblock to slant trans adapters used in trucks and vans of the day, before they made a 727 case specific to a slant.
Lou Madsen ("Dart 270" on the /6.org site, IDK if he comes here) somehow adapted a GM automatic OD (I think it was a 200R4) to a slant 6.
The wider gear spread of the 998/999 helped off the line acceleration and the lockup converter (if it works, Mopar had lots of issues with the early lock ups) can save fuel under cruise but won't make much difference if you mostly drive "in town" below ~40.