Keep getting P 0300 and P 0316 codes

Question

I changed the plugs, (they didn't look bad). vehicle only has 26K miles. engine heavily modified

Answer 1

The Ford MACH 1, the octane for this has to be a minimum of 91. and this not optional it is  required., when you set a P0316 it set and with a P030X which is suppose to identify what cylinder is misfiring , Because it is P0316 and P0300. this would point to the EGR. partially open EGR would set a P0300, or a partial plugged EGR passage would set a P0300. (your heavily modified)