I think shop manuals remain silent on subjects like the dowels because it was not in the realm of Chevrolet dealers to do total rebuilds. 50 years ago there were hundreds of engine rebuilders around the country and GM considered them competition because they not only did the labor, but they installed aftermarket parts. GM wanted to sell now short blocks. And then there was another competitor of GM... the junk yards. Being a licensed salvage dealer for many years put me in that category. The Hollander states that casting No. 3759365 has the dowel. It clearly states that the doweled block fits all, but "Do not drive in the plug above the oil filter inlet hole. When driven in it is to be used with a full flow oil filter." The 261 was designed to be a truck engine, but the full flow setup just didn't work in trucks that were worked hard because of the rubber hoses under the exhaust became brittle from heat. 261 engines were very popular in their day with the guys that pulled cable dumps, but a lot of them bit the dust when they lost their oil.
