Having long boney fingers is an advantage installing those header nuts....sausage fingers and you will struggle. I'm not sure why the plate is not sliding up to the studs and fitting for you. Ensure the pipe is fully in the head and rotate the plate and pipe if needed. For the nuts you mentioned you already have a universal swivel joint that you can attach the socket too...correct? Is it 1/4" drive like the following? Put that swivel and socket on maybe a 6" extension and try to thread the nut on by hand with the nut in the socket. You can apply some tape around the universal joint if yours 'flops' around too much. Otherwise, find someone with long boney fingers to start the nuts and finish with the universal socket. It can be a fiddley job if you don't have the experience. You english is very good.