I have a 1984 f150 with 300l6 when I crank it up the oil pressure goes half way and then when I start driving it goes all the way back down to the L and stays there just wondering if oil pump is going bad

 

With it building initial pressure i dont think it would be the pump. Seems like one of the tolerances somewhere is expanding with the heat and maybe loosing the pressure. How long since a change? Look all around engine compartment for fresh shiny oil, drip, sprayed, or a leaky trail. Get it warm then shove a large piece of cardboard (fridge box maybe) under it. Try to cover from radiator to atleast some of the tranny. Find the drops then try to find leaks. Look in aircleaner to see if its being pushed up through the head. Pull the plugs . See if one of them is oil soaked. Could be rings.