That valve looks like a square body style valve. ACDelco 467513. There are 3 port versions for fuel pumps without returns and 6 port versions for pumps with return. Looks like the 6 port versions with single wire are no longer available though. A later 6-port version is available but will require re-wiring.
https://www.rvpartscountry.com/Motor...Valve-Kit.html
Or just go to a manual style 6-port valve like a Groco Fv-65038.
My question is do you really want to use the aux tank? If you don't drive it a lot then the fuel gets old fast. On my '83 K20 I used to have dual tanks but honestly even road miles I need to stop long before I run out of fuel. If you don't need or or just want it to look correct it will be a whole lot easier to make the main tank work.