Ford F-150 surges, unsteady idle, almost backfires

Got 1988 ford f15o with 140,000 miles. Problem started with surging/sputtering at low speeds and rpms. Stepped on gas it was fine. As soon as done revving engine it would sputter hard almost backfire. But never died. Drove it for 2 days. Then parked it for about 2 hours came back out to start would not start. Then started for about 40 seconds ran real rough then stopped have not started since. During the initial problems the exhaust would have a gray/blue and sometimes dark gray smoke everytime stepped on gas. My mechanic told me possible catalytic converter blockage. Due to age, and extreme discoloration of exhaust pipes forward of cats. Don't know what to do.