If the steering wheel is not spaced away from the steering column surround, the back of the steering wheel will rub against it
, when fitting the wheel you need to make sure it clears in both the upright position and fully down position.
This can be achieved by undoing the top nuckle joint of the steering column, 13mm, I found this out after i changed my PS rack for a non PS one
I don't understand how you have fitted the wheel without the boss kit, unless somehow the old steering wheel has come away from the old boss and you have drilled holes in it to match the Momo steering wheel.
Normally the whole wheel comes off inc the boss, exposing a shaft, this is achieved by undoing the large nut, I think a 36mm and the use of a steering wheel puller. That tool pulls the wheel from the stub, you could try gently tapping the exposed stub or rocking the wheel up/down / side/side to loosen it, mine have just come off with no trouble. The new boss and wheel should slide onto this stub until it can't go any further then tightened up. If the wheel does not move freely do what I said above.
As others have said, we need a pic