hi
here some pics of the shaft to lock it in first gear. modded with four pins to hold the gear.
i would not lock it in 2nd because you get a very long gearing.
if you lock it in the first gear imo you still have plenty of options with the pinion (9 to 25t) and the spur gear (47 to 53t) to play with the gearing.
just my view.
bye
rhylsadar