This link shows the side of the block that has the oil holes on your 261, You will need to scroll WAY down to see the side of the block, you will see the large 1/2" tapped holes, next to the most forward 1/2" hole you will see this boss with the pin above or right along side. This pin needs to be pushed in 1/2" plus to route ALL the oil out through your spin on filter and then back to the oil galley. If the pin is out and flush with the block the full flow filter will do nothing. You can run the engine with the pin flush and no filter, pin pushed in and no full flow filter ZERO oil gets to bearings.

http://www.oldchevytrucks.com/blog/index.php/category/technical-articles/1955-66/mechanical-1955-66/

Last edited by JiMerit Boltr#43; 02/05/2017 12:56 AM.
It's easier to get forgiveness than permission!
1946 1/2-Ton Chevy
1953 Chevy 3/4-ton Factory Stakebed