Oil pump pressure / pressure regulator

I'm having oil pressure fluctuations after about a hour of steady highway driving in my '77--drops from ~50 to 30-40psi, floating between the two even when there's no change in engine speed. I already replaced the sending unit, which reduced the fluctuation but didn't eliminate it (it was ~25-50). If I can still get a new pressure regulator valve assembly (#15102-21005) can this be replaced with the pump still on the engine? Or is it even worth going this route since MSA sells new pumps for $50?

On a side note does anyone have experience with the high pressure oil pump springs MSA sells?

Thanks!