I have a 241HD and was just planning ahead "Just in case". As you know, NV5600's were very new in '99 and I have the NV4500, BUT I always have my eyes open for options... If the 5th gear nut ever hits me I think I will try to convert to a 6 spd, instead of fooling around with fully splined super wazoo etc. etc. 5spd. In the end it is still just a NV4500...