Patrick, OE pumps are listed on eBay all the time, some NOS, some for reasonable $. Search by the AC part number as well as Chevy PN, and you should see some come up.
There are several AC pumps that might apply to your 261. A few of them are different only in the angle of the inlet and outlet fuel line connections. If the truck still has the '53 fuel lines/orientation, the AC numbers would be 9797 for a combination pump or 9798 (superseded by AC 4149) for a single action pump. If the fuel lines/orientation have been modified for a '58 or later fuel pump, the PNs are AC 4666 for combo pump, or AC 4434 for single action.
Some of the more recent pumps with those numbers will not have the glass fuel bowl. True NOS that are old enough will. With NOS, a rebuild is likely needed, since the original diaphragm may be degraded, and not E10 compatible, anyway.
As far as rebuild, they are a little trickier in a few areas compared to carbs (such as removing and reinstalling the staked rocker arm pin), but overall there are fewer parts, so not complicated by any means.
I have a 24-page AC Fuel Pump "Shop Manual" dated 11-57 that I have scanned and can make available as a reference for anyone interested. It's a little generic, but it covers the basics of disassembly and reassembly, as well as troubleshooting and testing for both single action and combination pumps.
Last edited by drdoug; Mon Jun 01 2020 02:56 PM.