Yes, Imperial Services offers such a conversion. You send in your pushbutton shifter assembly and they modify it and send it back with some other parts so you wind up with a single cable correctly pulled and pushed by the pushbuttons and park lever to operate the one and only shift selector lever on a '66-up rod-shift transmission. It is costly and takes forever, that is if you can get a response out of Imperial Services to begin with. :roll:
Do you really need to go this way? Seems to me you'd have an easier time obtaining a stoutly-built cable-shift 904 all ready to go and fully compatible with your unmodified button box.