Re: When replacing the impeller in a Mark 58ae...
I've replaced impellers in a bunch of those (or very smilar) models and just replace the impeller.
There are at least two problems you might encounter.
The water pump cover is a round disc with two round indents in it for a special tool. A pin spanner wrench may be used. Sometimes a propane torch and spray lubricant can help things along. If it's frozen in plac badly, a hammer, and punch may loosen it but will ruin the cover. If it's still stuck they sometimes can be removed in pieces with a dremel tool, or chisel. The new covers are available at oldmercs.com. Don't cut too deep or ruin threads in housing.
When assembling I prefer to have the lower unit, and shifter in neutral. The lower unit will "pop" into gear as you try to do this. Afterwards you will have no neutral or reverse. Take it apart and do it again till you get it right. The shift shaft must be rotated all the way around in one direction only to "find" the detented gear positions. Do not pull up on shift shaft or it will come out of shift cam internally and can be a big pain to fix.
There are no gaskets involved when doing the impeller.
