Hello, GW,
Unless your 1961 235 engine has been converted to full-flow filtration, you have ports for a by-pass oil filter.
Flow direction:
http://www.1954advance-design.com/Web%20images/blockoil.jpgIf you have holes/ports on your block in those locations, you have a the original by-pass filter capability/set-up.
Have you looked in the Stovebolt Tech Tips section?
https://www.stovebolt.com/techtips/In particular, the excellent/extensive Tech Tip on by-pass oil filters:
https://www.stovebolt.com/techtips/engines/oil%20filters/index.htmlThere are installation instruction near the bottom of that page.
Place a Parts Wanted ad here, or check out eBay, or Jim Carter Truck Parts, Chevs of the 40s, or Classic Parts.