Chevrolet Canada,
If your 261 has only the water pump and generator pulleys to run off the crankshaft, a web search I did says an "ACDELCO 17440" (commonly known belt size C50) should fit your 1960 261 engine.
That's assuming (yes we all know what that means - lol) that the generator mounting point and brackets on the engine are the original 1960 261 units and not something else.
Others will be along to add info or correct me if I have strayed on my advice.
Let us know what you find and works please and thanks.
Dan
EDIT: After going back and looking at earlier posts in this thread, did you add a relocation plate to the water pump??? If yes, that will cause the suggested belt to be too short.
Last edited by Gdads51; 08/03/2023 12:19 PM. Reason: update info