have an 89 conbrio with the 351.
in the center of your hull just in front of your engine is a large drain plug.
Open it and drop down thru it the oil drain line that is connected to your oil pan. This line will have a fitting on the end of it.
put your drain pan under the hull and centered under your oil line that is dropped down. Open the fitting and voila--your oil will drain out! Change your oil filter, button up the oil line when done draining and make sure to put the drain plug back in place!
easy done!