Keep D8K cool.
I hate to even suggest it, but have you actually dipped a thermometer in the coolant to see the actual temperature. I've seen more than one Cat have a lot of expensive work done to it just because the temp gauge was faulty.
Presume you checked the fan drive pulleys for wear. Cat belts are made to run high on the groove and the pulleys are actually machined with higher sides than normal. If grooves are badly worn however, and belts are riding way down in, the pulleys are worn out and need to be replaced as the belts are likely slipping. Sometimes you will hear a squall and squeal as you accelerate and de-accelerate the engine. Best to replace all three pulleys (drive, fan hub, & adjuster) together with new set of belts. If you shut the engine off, and the fan keeps turning for a bit, they are really worn out.
Dpete's concerns on the rad are very valid. If it wasn't taken apart and rodded out, there may be blockage in some tubes, enough to drop cooling capacity 20-30%. Proper coolant temperature regulator is very important. Don't run without one!!!..... Older Cats were built with excess capacity in the form of tubes in rad assembly, but over the years as repairs stack up and tubes are blocked off, that extra disappears. Silica Gel blockage has been a big problem in the tubes, and mostly can only be removed by disassembly and rodding out, followed by a good soaking in the hot tank. External core and fins must be clean, and straight. Sand grid must be clean.
Oh yes, is the belly pan empty? If you have by some chance run in the mud and the bottom of the engine is buried in mud, oil and grease plus Twinkies wrappers, this will cause heat buildup as the cast iron can't radiate away the excess. 1" of mud equals 1/8" of steel so far as insulating value is concerned, if I remember right.
Deeper matters; if there is a problem with the trans, it may be overpowering the trans oil cooler. Small tubes in this cooler are easily plugged. Same for hydraulic oil cooler if equipped. If brakes are dragging for some reason, or if there is some type of drag in the drivetrain, this can also cause excessive heat that the cooling system can't get rid of. The engine oil cooler is bigger, but bits of rust, silica gel, o-ring and other foreign matter will plug it causing oil temp to go up. Hopefully, no one left a rag in the system causing a blockage. have seen this a few times.
Let's see, you mention that the old girl was rebuilt in the last 1200 hours. I'm again presuming all genuine Cat parts were used? These engines ran sorta loose, but if non-OEM parts were used, they may be tighter and not worn in yet. More drag, more heat.
Just a thought, suppose all new coolant hoses were used and the pressure valve is new? Older hoses could collapse and cause some restriction. Almost forgot. Caterpillar has had more trouble with water pumps than anybody else I know. Impellers coming loose on shaft mostly due to loose or busted retainer bolt. Sometimes they still turn but actually slip on the shaft slowing down coolant flow.
On to the fuel system; Was it rebuilt and properly set? Too much fuel, lots of extra heat. Lots of black smoke, very slow windup, may be a bad turbo and again, lots of heat.
Gad! There are a zillion things that will make an 8 run hot. I just can't remember them all, but hope these help.
Good Luck!