changing from auto to manual is really easy (especially is your going to swap the motor). i'd say go look at the install guide at
http://www.240sx.org to see whats all involve and if you wanna do it. im about to do it to my 95 240, im going to start getting all the parts for it at the end of this week and ill get it in there in about a month... hardest part is finding all the parts... putting it in is not much harder than putting in a new clutch. id say with two people, alot of jack stands or a lift, and all the necesarry tools... you could do it in 6 hours of constant work or a whole day for you lazy ppl