I guess not all frame rails had that hole cut in them. My 3 e-bodies have that bracket welded in so I can't provide a measurement of an unmolested hole. You should either cut a hole for your tab to sit in with a dremel cut-off disk, or cut off that little tab before welding the bracket. It should fit flat on the rail. The big nut on the outside pivot ball is centered inside that access hole. It will be hard to put a socket on it if it isn't lined up. Install that pivot ball, with it's lockwasher, put a socket on it, sit it in the access hole to center it, then tack it in place. That bracket face sit square to the z-bar. You might want to wait till you trial fit your engine and trans, to tack it in. Pull everything back out, then finish welding it in.
There is a lot of pressure on that bracket, depending on your pressure plate. Don't put a weak weld on it.
