The 60-61 truck resto packs are where I found a reference to RPO 311, but no designation about what it referred-to.

http://www.1954advance-design.com/chev-specs/1960-t-ser.pdf
http://www.1954advance-design.com/chev-specs/1961-t-ser.pdf

The 1962 resto pack is where I found the RPO 311 referring to the truck 235 with Powerglide. But, no Engine Serial Number designations is in that resto pack.

JB was the truck 235 with Powerglide designation in at least 1961 and 1962:
http://www.1954advance-design.com/chev-specs/1962-t-ser-ID/