From the link I shared above:
The Canadian Pontiac 6 was based on the truck engine but had significant differences.
Most important was the fact that the Pontiac was 'a step above' the Chevy so the common 235 would not do, hence the bigger better 261.
"Basically, this larger engine was a 235 with the same crankshaft but GM engineers made various modifications to give it extra strength and horsepower.Its standard bore diameter increased from 3-9/16 inches to 3-3/4 inches. The connecting rods were heavier and attached to increased diameter piston wrist pins."
So same stroke, bigger bore.
Also, "Its higher lift cam shaft, for better breathing, was shared only with the early 235 six cylinder Corvette. A modified larger Rochester carburetor was also a 261 only feature. Unfortunately most of these larger sixes have long since had their original Rochesters replaced with 235's and therefore do not perform to their full potential."
Keep in mind: "the 261 truck engine came standard with solid valve lifters and an aluminum camshaft timing gear. The Chevy passenger car's 235 was equipped with hydraulic valve lifters and a fiber timing gear for quieter operation.
During the 1955-62 Canadian Pontiac application the lifters were the hydraulic type, the cam gear was fiber not aluminum, and it did not have the full flow oil filter. These Canadian made 261's did not add the full flow filter in 1958 as in the U.S."
