Compression Check low oil pressure Now What?

The original 318 in my 1972 Swinger has 125k miles on it. I'm the third owner BTW. The car did sit alot for the last 4 years until I just got around to driving it this year. I'm having a lot of valve noise and I'm sure this is due to low oil pressure. When the engine gets hot, at idle I have 3-4lbs oil pressure and never above 30lbs when driving. I tried 20w-50, 10-30 and 10-40 but no change. The compression readings are 135-140 on all cylinders. The car doesn't burn any oil or smoke while running. It does puff a little smoke at cold start up sometimes.

So I'm wondering if changing the oil pump will get me another few years or just cause more issues? How hard is it to change the oil pump and probably the rear main with the engine in the car?

Craig