The stock gauge has a range appropriate for a low pressure splash oiler while your 261 is an pressure oil motor. You probably aren't going to get accurate meaningful oil pressure readings from the stock gauge with this setup.

1955 1st GMC Suburban | 1954 GMC 250 trailer puller project | 1954 GMC 250 Hydra-Matic | 1954 Chevy 3100 . 1947 Chevy COE | and more...
It's true. I really don't do anything but browse the Internet looking for trouble...