I'm actually doing that step now.
I decided to go by the instructions and rivet the J stiffener as I get down there with the rivets. I left it completely out for the first 4-5 rivets from the rear spar than added it (when I knew it was now or never).
Before riveting the bottom skin, I clecoed the J-stiffener in place just to see how it'd affect moving the skin out of the way for riveting the rivets closest to the rear spar. It locked the skin in place and made accessing the rivets closest to the rear spar even harder than it already is for myself and my short arms.
My advice is to leave the J-Channel on the work bench until you're about 4-5 rivets down the ribs from the rear spar. Then put it in place but don't cleco it until you can easily access the rivets (it does get a lot easier the more you advance toward the main spar. Once convinced that you can reach the rivet tails, cleco it in place and rivet down the length. I have my 96 year old father shooting and I'm bucking but honestly, most of the J channel I did by myself and it looks great. I was about ready to sell my kit riveting the skin to the rear spar (not really, but it wasn't pleasant). Its one row and definitely gets easier as you go. Good luck!