lexun,
The oil cooler and adaptor, if installed correctly, will not be the cause of low oil pressure. Though warmer temperatures make 20W50 a good idea, 10W40 oil is also not likely your problem. In forty years of this work, I've yet to see a stuck oil pressure relief valve in any but a weathered old rusted block, but if some kind of crud got in there I suppose it could happen. I wonder about the experience level of the previous rebuilder. There are a couple of "rookie" mistakes that can cause low pressure like this.
Improper placement of the gasket sealing the oil pump body to the block can do it. So can a faulty or out-of-spec oil pump not replaced to save a buck. (bad idea) No matter what, if this noise is now present, you have likely damaged the lower end of the engine. I'm not there to hear the noise, and that's always a handicap in diagnosis, but if the pressure is that low, and the rapping is "palpable", I'm fearful you have a bearing failure by now.
Step one is to remove the oil sump and inspect the bearings/crankshaft. Get back to us with the results. We'll see what we can do.
The fuel running out the carburetter float bowl overflow pipe simply means the float valve is stuck. If it hasn't been done in some time, you really should rebuild the carburetters. Check the throttle shaft bushings (not choke shaft) for sloppiness. If they are sloppy you will need to have them rebushed as well.
Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon