what brand of engine coolant should i use.plan on changing fluids and flushing radiator soon can u use peak or preston or is there some thing else required.

That's just fine... Most would suggest a pre-mix coolant because it mix with distilled water already. Or you can mix it yourself with distilled water. I've been using Napa, Prestone, Peak what ever is on sale. As for flushing... http://mopar.mopar1973man.com/cummins/general/cooling/cooling.htm