from my recollection it not important what gear position the shifter lever is in rather making sure the receiving forks are aligned properly. Obviously this is done with the housing off then just play with the tailshaft housing. If when its on & all gears shift, you're done.

I know that upon reassembly I didn't even think about shifter position I just aligned those forks & played with it until it went in properly, took a few trys. I remember pulling it apart once & having to clean up the RTV silicone.