440 Oil Pressure Issues..

Yes, that is the relief, but it is on the pressure side of the pump not suction, so the theory of an air leak doesn't work. However, backed out reduces the spring tension and therefore oil pressure. You need to remove it, the spring (note any shims installed with the spring) and the poppet valve. Polish the poppet well, ensure it is all clean and put it back together. There is a copper crush gasket under the loose fitting.

Easiest way will be to gently support (lift) the engine from below with a piece of wood on the oil pan and bottle or floor jack. You might have to remove the engine mount for access.

Funny, yesterday i put the new (to me) 440 in my 55. The oil pump had been leaking so I removed it and changed the gasket and o ring. Once back in the the car I looked up from below and was surprised to see a drop of oil hanging off that same fitting. When I reached up it was only finger tight...... mine is much more accessible though.