I have a 1979 904 tranny that I want to mate with a 1965 slant 6, is this fine to do?

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mopar or no car!!!

you can do it -- but this is not a cheap option
1) your 65 rear crank register is the small one
your 904 has a large style register

2) solution ---machine- crank register or swap to a 68 & later crank
or order a special torque converter with the correct dimensions

3) go to Lou's house he is giving away the tranny you need.

4) someone else will chime in with spec -- but this should give you an idea of what your up against

This is a combination that does not "bolt together" without some major "re-arranging". As noter, if you use the 66-67 "small input shaft" 904 you can use the 65 engine.
Also be sure you swap the oil pan and pick-up tube off the 79 and onto the replacement engine.
DD