A 261 and 235 are part of the same engine family. You can tell if its a 261 by two cast in vertical bars above the starter. Sometimes referred to as Captains bars or the number 11. I will try to find a picture. Also be aware of the big truck 261. It will have a full flow oil filter with very large 3/4 inch oil lines. The full flow filter cant be removed, if you plug the ports in the engine you will have no oil flow to lubricate the engine. This should help.
Ok....founds the Captains Table on the 59 Stratochief.......but not...on the GMC.....but Transmission on the GMC looks to have been changed....so maybe engine too?
The 235 has an oil filter can...on the left side.......Were these a good thing?
I heard on the earlier engines they restricted oil flow..
Yes and most Canadian 261 engines start the serial with a W before the numbers. I have the original 261 in my 55 with almost 104,000 miles. I also have 2 other 1955 261 engines.
Al