Hey @Toga,
I just replaced my window rail this afternoon and have a few tips that might help if you haven't already gotten it installed.
The new OE unit i s certainly much narrower and I couldn't get both the gasket and the glass in without forcing things. What I ended up doing was clamping the new unit to my workbench and working the channel open with a wide pry bar. Not the most elegant approach but it opened it up ever so slightly where both the gasket and the glass fit in snug.
It'll be a bit of pry and test fit. You won't need to crank it wide apart. Hope this helps.