You are looking for this manual "Service Manual Mercury Outboards 1965 and Prior : C-90-25500" Shop around for the cheapest price.

The WH-6 is supposed to be similar to the Mercury Mark 6.

The PDF of the only free manual is available:

Go to: http://search.ebscohost.com/

Log in with:

User ID: marshall

password : public

-----Go under the Small Engine Repair Reference Center (look for the wrench), then Marine/Boat Motors....you will click on Mercury in the columns and then click on Mark 6 or Mark 6A.

Oops, I spoke too soon. A similar manual is available here:

http://www.fiberglassics.com/library/File:Mercuryservice001.pdf

Mercury Mark Manual can be found at:https://public-mercurymarine.sysonline.com/Default.aspx?sysname=NorthAmericacompany=GuestNA_KEY=NA_KEY_VALUElangIF=englangDB=eng

That is at: Mercury Marine - Parts and Catalogs - Outboards - K Series.