Are you aware that a 261 could have come with full flow oil system and that the 235 was always bypass oil filter if any?
Also the 261 can be changed reasonably simply to use a full flow oil filter.
This means that if your 261 was or will be configured for full flow oil filtration the old 235 bypass oil filter is not correct and will cause engine damage if used in place of a full flow filter.

Or are you just trying to install a bypass filter alone on the 261?

Do you have pictures of what you're working with?

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-