318 running bad

A new timing chain would be a real good idea if it's the original. Anything with that many miles has to be pretty worn and can leave you setting at any time like MoparJ said. That may not be the only problem though since it has so many miles on it. Run a compression test and that'll tell you how sound the rings and valves are. You might have some leaking valves. If the compression test shows decent I'd look at the carb. next. Might have sucked up a piece of dirt and have one of the jets partially clogged.