I called Penray and left a message with the question about 2000 or 3000. Their website is pretty specific about 2000 is for plain water systems and 3000 is for systems that use a glycol coolant. It does also mention 2000 is for diesel engines but does not say that it is ONLY for diesel. I’ll let you know as soon as I get a answer.
As far as your cooling issue, is it just when it’s idling? How is it when cruising or particularly pulling a hill. You mention good flow so it is less likely, although not impossible that the system is restricted.
Starting with the easy stuff, the first thing I would look at is if the fan belt is loose or is glazed and slipping. A quick check is if you can turn the fan easily by hand when the engine is stopped, the belt may be slipping. If you press on the belt midway between pulleys it should deflect 1/2 to 3/4″ with a light press. Be careful not to over tighten. Also, are you running it with the timing advanced and is the timing set right? Retarded timing can make it run hot.
After that you’ll have to start looking at problems with the water pump or restrictions in the system.