It is a 155mm pump, only size that came on the 951's.
But to answer your question, page 13-02-3 of the factory service manual, Impeller diameter 155.6mm.
No, it uses a rubber sealing ring around the pump but I think the parts diagram still calls it a neoprene seal. I can’t remember if the 98’ uses it though as that one year is a little different on a few things.