Well, boy oh boy, did I get leveled! Never is a powerful word, and certainly positive.
The first problem is your statement about the 261 engine. In the US, it came out in 1954, and was an overbored 235. The bore went from 3.5625 to 3.75. It is good to know that in Canada they increased the crankshaft stroke, as you said it is a "stroked 235". Wonder why they did that? You would think they would save the money and use the version they developed for the USA.
Next, the never part. As a college student in the late '50's, I was the product placement director (lot boy) at the Chevrolet dealer in my home town. I cleaned up and prepped out medium duty units with 322 inch Buick vertical valve V8's in them, from the factory. Later, in 1970, I bought my 55-2 GMC from a farmer in Rigby, Idaho with a frozen up stroked 235 in it (funny, I always thought it was a 248/270, never did check cause it was RUINED
by pure water being left in it over a -30 degree Idaho winter). I swapped in an early (303 CID) Olds V8 I had available, and took the result past the GMC dealership who had pointed out the farmyard where the truck was sitting. He looked at the Olds under the hood, and said, you know, GMC put those engines in their trucks back then - and led me to the trade in lot where there was a mid-fifties GMC 2-1/2 ton spud hauler truck with a by-gosh genuine 370 cubic inch GMC V8 engine with valve cover bolts thru the rocker covers centers - sure looked like an Olds to me, and I guess the late Bill Treasure should know - he was the largest GMC dealer in the Intermountain West.
When I went to document this data, I could not find much on the internet. I found a reference to one in an engine color site, and another in the Egge piston site. So, I went to Rob English, whose site is OldGMCTrucks.com and he spent some time in his huge collection of data books and came up with documentation that they WERE used - and stated, that through all his years of studying GMC,s they were new to him, and that you learn something new every day.
From what I know, your data about the 60 up V6's is accurate, and your data about the Pontiac V8 in the 55-59's is also accurate. Canadian vehicles were handled differently, and I can well recall a visitor in the repair lanes of the Ogden, Utah Chevrolet dealership in, I believe, late 1959 with what looked exactly like a '60 Pontiac, except the wheels were way inside the fenders (unlike the "wide track Pontiac's of the USA) and the hood was open and the familiar Chevrolet 235/261 6 cyl was under it - a real eye-opener. I think it was an Acatian, or some such name, had Manitoba license plates.