You can leave the bellhousing crossmember along with the original bellhousing and just hang the T5 on the back of it. Yes you will have to have the driveshaft shortened. As long as your differential gears are 3.73's I would leave them alone. Changing differential gears is not something easily done for an inexperienced mechanic and is quite costly to have done professionally.
There is a member here that knows all there is to know about the T5's and what you will need. His user name is Lugnutz65. Here is one of his threads. His website links are in his sig line.
http://www.67-72chevytrucks.com/vboa...d.php?t=612147