The Motors Manual tells you how to set the timing, not the timing specification in degrees TDC, BTDC, BUDC, ATDC, AUDC.
Flywheels get changed, especially if a 12v 261 flywheel was swapped for a 6v flywheel, or if a 216/235/261 flywheel get interchanged on a different size/year engine.
A swapped flywheel might be from a 216 or 235 or 261. In the same year, a 235 and a 261 might have different timing specifications but the same timing method/instruction (set so the pointer lines up with the ball on the flywheel).
Tim, what year/engine flywheel is on your 261? And, how did you set your initial timing?
Checking out your cooling system seems to be a good place to start.
