The "p" heads are not drilled for emissions, but since you have a '79 it wont make any difference.
I would reccomend getting a set of the Ford Racing "p" head headers. They are mad specifically to fit the "p" head. They will really help with the clearances around the spark plugs and save you alot of aggravation. I have a set on mine and have had no issues with the plugs or plug wires.