I made a tool out of an old impeller to press the seal in to the correct dimensions. There are rebuild instructions on the flathead techno-source site titled "49 - 53 waterpump rebuild by Dave O'Neil".
http://www.btc-bci.com/~billben/flathead.htm They are a pain to rebuild but the kits are around 25 bucks if you have the tools & equipment to do the job.
Kerby