My brother-in-law just acquired a 63 Dodge dart slant 6 904 push button transmission. The transmission struggles to switch gears. Would it be best to switch out to more later model 904 or rebuild the original. In my opinion I would find a cheap later model 904 and install a aftermarket shifter.
I'm hopefully in the final stages of same dilemma and finding knowledgeable experienced folks to do the work is pert near the biggest challenge. Two cables for shift and park mechanisms plus availability of cables is another concern.
I'm gathering parts to go to a 4 speed and be done with it eventually.