Frank,
I have had the same problem with parts clerks. Most of them will not take the time and trouble to help find a part if it doesn't just jump up on their coumputer. Almost any part is available if you can find someone who will take the time to look for it. Try to find some one who has been in business for a long time and has kept his old part books. Usually if you can find some one who has a collection of old dirty books under the counter, he can find the part number and then begin to try to interchange it. I have been luck enough to find two places like that, one is a NAPA store, and the other is a CarQuest location that has a machine shop. The shop foreman has been at it for a long time, and is willing to take the time to look for things. Another place I have found parts is at a local hardware store that carries a very large selection of mechanical items. They have a section that handles bearings and seals, etc., and can often find other old unusual items. Have you done a search of Junk Yards? I have been lucky with that in the past. I found one on the internet that had a number of parts for a 6400 I am working on. I don't have the name at hand but will try to find it and send it to you in another post. Sorry this got so long, I seem to ramble. I guess old age is really setting in. Good Luck Allen