Pardon me for resurrecting this thread, but I'm having a related issue and Pony66 seems to have some of the info I'm seeking.
I'm working on a very early '64 289 with a C3OE-6059-B timing cover. It had an aftermarket stock style aluminum pump installed on it that was causing the water pump pulley to be about 3/8 of an inch too far forward. This created belt alignment issues with the WP, but the A/C and P/S pulleys line up correctly.
I have deduced that the pump is the incorrect one for this application, and I believe the iron one with no backing plate pictured above may be the one I need. The question is... where do you get one? All the iron ones I see at the parts houses have a backing plate.
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Any help is appreciated. Here's a pic of the style of cover I'm working with for reference...