you do need a manual cross member. if the one the guy gave you had a "b" stamped in the center it's for 5-speed..if it has an "a" it's for auto..
also, here is a link to a thread that specifies lengths of drive shafts. measure the front portion that you have and see what you find out.
Driveshaft Lengths - Nissan 240SX Forums
if you have more questions let me know