Almost impossible to be related to oil pressure. If this were the case big ends would fail way before a lifter rattled. My guess is the engine is pinging due to a lean mixture or timing issues. Have a good spray around the intake manifold and carby base with some carby or brake cleaner. Engine will drop rpm or stall if there is a big enough vacuum leak. Any noticeable drop in rpm is a problem. Also make sure you have the vacuum advance line removed and blocked from the distributor when you are checking the timing. Also make sure that the vacuum advance is working and that the mechanical advance is working. Engine idle speed should be set by adjusting the anti dieseling solenoid not the adjustment screw on that dirty old varajet carby. The adjustment screw on the carby is for setting fast idle speed in relation to the choke. And yes camshaft has to be replaced at the same time as lifters. Like not wiping your a#*e after going to the toilet. You just don't do it. Throw the varajet in the bin where it belongs.