It shouldn't be too bad as long as the transmission has the mounting pad for the shifter on the tailshaft housing. Either find a stocker or put an indy 3 speed shifter on it.
If there's no mounting pad to bolt the shifter to the transmission there were "universal" type shifters available back in the day but these were a "one size fits all" type deal so you might have to fab some stuff to get one of those to work.