I'd suggest you get some KR in 35mm format, make lots of identical exposures at ISO 64, and process them in different ways, to see which method yields the best results for you. Then use that for your 120 film.
Some things you need to know about: Kodachrome has a remjet backing which needs to be removed either before processing (preferable, if you can manage it in total darkness) or after. Secondly, there's a yellow filter in the film that you cannot get rid of if you process into a negative. Thus the negatives might be hard to print from optically, but they should scan fine. If you do reversal processing, the yellow filter will go away.