just some issues

I own a 1990 buick century with the 3300 V6 in it, it has 198,xxx miles on it and so I changed to timing chain and gears because it was making all sorts of noise and getting like 8 MPG so I changed it put everything back together and filled it with coolant and oil and new filter, started it and it didn't hold oil pressure, so I take it all back apart and put a new oil pump in it again fresh oil and new filter and fresh coolant........nothing it has a new timing set, and new mellings oil pump and it holds nothing my oil light dims out right as I start it and then it turns back on, any help here.

Are you getting any unusual noises when you start the engine? This sounds more like an issue with the sending unit than the pump especially if the engine is quiet. I would try another sending unit or a manual guage before suspecting the pump or the engine itself. If the engine is making a lot of noise then the pump may not be functioning. The oil pressure is determined by the cam bearing clearance and it if is too great then there won't be any oil pressure. You may end up with new cam bearings. One question, did you prime the pump with vasoline or other grease before installation?

vasoline, I packed it full

Well just out of curiousity I went out to it today (after it has sat a week or so) and started it and it held pressure for about 20 seconds and then it quit holding again, so I'm going to try to find a wrench big enough to fit the sensor and I will change it but before I put the new one in I'm gonna turn the motor over to see if it pushes oil out of the sensor hole, and other ideas that I may have overlooked?

Not really! You have done everything correct.