318 Retards Under Load?

1st place I'd look is the gas. Pull the fuel line off & take a clean glass bottle & fill the bottle 2/3-3/4 way up while cranking the engine(it helps to have a partner do th cranking so you can watch the bottle) & then let it sit & settle. Any water or contamination should separate in the bottle. Important: DO NOT USE A PLASTIC BOTTLE since the gas could disolve the plastic & risk leaks &/or a false sample. Take a smell of it too. Make sure it doesn't smell like diesel fuel or kerosene. If the gas is ok check your fuel pressure & volume. Next look at the ignition system- cap, rotor, plugs & coil. Try to be systematic in your work & not just start throwing parts at it. Keep us posted on your progress.