If the water ballast dump valve is open or the airbrakes locked, assembly can be difficult.
If the flaps are in L position it will go together but the stick won't move side to side and the flap handle won't move.
MM Fabrication sells an assembly tool (designed and originally sold by Steve Bralla) that works very well. A more simple approach than tapering the pins. It wasn't on his website last I looked, so call or email Mark.
Jim