I have a 55 1st series with the same problem. It has good oil pressure when it starts and as it warms up, it goes to 0. I don't drive it too far or too often but have driven it as much as 40 miles and it never makes any unusual noise. I originally did the things you were talking about. I checked a different gauge, and even put a new oil pump in but it did the same thing. I have finally figured out that there is too much clearance somewhere. It does have new mainbearings with the right clearance. I hear that it is probably rod bearings or even cam bushings. I just hate to pull it out when it is running so good but I also hate to ruin everything internally.
