In 1967, a solid fuel rocket motor with 260 inch diameter and 89 inch throat dia, was tested, and it was found to generate 5.8 million pounds of thrust. This was much higher than the F1 engines used in Saturn V. Why this motor was not considered suitable for Saturn V application?
Rocket and rocket engine development takes years. The F-1 was first test-fired as a complete unit in 1959, and completed flight rating tests in 1964, and the first Saturn V flew in 1967.
Aerojet's 260-inch SRB was first test-fired in 1965. The third test in 1967 unexpectedly ejected the rocket nozzle toward the end of the burn, spewing corrosive material over a wide area. It simply wasn't ready for use by the time the last Saturn V had been ordered.
The 260-inch booster was considered for a number of post-Apollo variations on the Saturn V design, including the Saturn V-4/260, which would use four of the 260-inch SRBs as strap-on boosters with a strengthened Saturn V first stage and uprated second stage, which could have put ~360 tons payload into LEO -- almost 3 times that of the Saturn V. None of these concepts were developed past rough design proposals, as far as I know.