If the motor was rebuilt with forged pistons, those have a higher thermal expansion and are made with a higher cold clearance, so will rattle until operating temperature is reached.
Bore wear could be a factor but ought to come with symptoms like oil consumption & smoke.
Off-idle stumble can be from a too-lean main circuit "covered up" by a rich idle screw setting. Vacuum leaks can be a significant low-RPM lean contributor. If car has power brakes, use the brake pedal's "booster air surge" on apply, to see if running too lean (brake will induce a "lean stumble".
A wideband AFR meter is a great tool for tuning carbs (essential for EFI, good idea for carb). You can get the main circuit jet / rod / whatever trued up at cruise, then adjust the idle RPM and mixture. A big cam will affect accuracy at low RPM but this is not mentioned. If all you need is a < / > stoich reading then a real cheap heated O2 sensor can be useful, you need cig lighter power and a multimeter. I've tuned mixture on cars from carb'd Chevy van to EFI Porsche 928 this way, back before WBO2 meters were commonly available.
Hey Sajeev: I think the better question is: "Does the noise go away as the engine warms, or is it always present?" Has anyone taken a valve cover off recently? If the valve area is caked over with old baked on oil, then it's really time to go to 4 quarts of 15w40 Diesel oil, as you suggest, plus one quart of transmission fluid ( I prefer Dexron 2 or 3). Then drive the car for at least one hour, not just idling, drive it! Then dump the oil. Do the same oil change a second time, while it is still hot. Once again use 4 quarts of diesel oil (Rotella is my favorite also, but I also use Mobil Delvac, when I can't find Rotella) and one quart of transmission fluid. This one can be driven for a week or two, but get it really hot when you're going to change it again and them go to a regular 5 quart Diesel oil change or use an oil that is formulated for Classic cars.
All of this assumes the noise goes away when the engine warms up. If it doesn't go away, the problem might be with bearings or something more substantial. There is also the possibility of a broken flexplate which can sound just like a bad rod bearing under a lot of conditions.