If its the main box you are fitting obviously check the clutch / pressure plate and bearing, , check the release arm pivot point for wear / cracks and make sure its greased (might be worth replacing the pushrod clip as it may be getting brittle and give you grief later, crank oil seal for weeping and the spigot bush in the end of the crank. Basically anything in that area is going to cost you a lot of time if it goes wrong so make sure it is good.