When you put the mainshaft in, put the endplate on, there is a tight spot, but only when you install the crown wheel and spider.
A. Make sure the crown wheel and spider are compatable, ie they don't bind up when mated together, but not installed. Could be bad crownwheel/spider brearings.
B. Check to make absolutely sure that there is no dirt between any of the teeth on the gears on the layshaft and mainshaft.
C. Or as Mark said, try another endplate.
As the problem only occurs when crownwheel/spider are mated, that's the best place- to start looking.
Apparently, cases and endplates are a matched pair, I seem to have read somewhere. Can't remember where I heard it
. Also apparently, there are some people who seperate cases and endplates and stack them on the shelf seperately 
