Installing a repair kit on an old toilet
What you have may not work. The Douglas valve or flush valve (The brass looking piece in your toilet) extend through the toilet and the Ell goes into the valve and the whole thing setts with  a nut. If the repair kits flush valve does not have enough to set, go through the tank, allow the ell to set and screw down with a nut, it is not going work. You may need to go to a plumbing store and get the correct kit for your toilet.