Not sure on the 250 but on the 500 trans you really need the cases together as it supports the dowel pins the shift forks sit on which keeps everything spaced properly and able to shift, otherwise they bind unless you hold them apart from each other. 

You can put a little type F or what ever you are going to use for tranny lube on it if you want but if you're worried about it not being done right I'd put the cases together dry with 2 or 3 bolts and make sure the tranny works then pull them back apart and seal them up with 1211 like they should be.