The water pump is attached directly to the accessory drive bracket with two 9/16" (1" long) bolts.
I removed the impeller for inspection (and because I haven't had a good look at the impeller housing before since my current engine's is crammed in the worst ungodly position in my engine compartment).
The impeller was in good condition but I will replace it since it is old. The impeller is held on with 4 flat head machine screws (1/2" long). There is a full description of each part in the impeller system on the Moyer impeller parts page. The o-ring gasket on the impeller plate is missing, or so disintegrated as to not be recognizable. I will replace that on reassembly.
There is a gasket on the drive side of the assembly. It will need to be replaced.
The Moyer site has a great page on the accessory drive. http://www.moyermarine.com/access_repair.htm
[All of the posts associated with this rebuild are available under the search label Atomic 4 Rebuild.]