I had something similar, and like you, there is a bit of pipe sticking out of the "T" to fix a 90* fitting on. Will look a bit naff, but you can use a few fittings and pipe to do a "round the houses" jobby, at the same time replacing the tank fitting....
PS. I also did this under my kitchen floor to fix a pinhole leak too, you know, the pump starts up every 5 minutes, nobody knows it's there.????