Broken rocker arm

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1972 350 with lots of miles that burns lots of gas, and oil. Oil pressure is decent when moving, but pretty low at idle. Recently developed a knock that goes away when the 2nd spark plug wire on the right, is shorted. Pulled the valve cover and found the base of the 4th rocker arm broken. Pushrod looks straight. Same noise with the broken arm removed.

1. I'm thinking I need to pull the pan and look at that rod/journal?

2. Can the pan be pulled with everything else in place? Auto tranny.

3. Is a broken arm typically of symptom of ??????, or just old age?

4. Is it possible to buy a single rocker arm, to replace the broken one?

Thanks!