Glad to hear you resolved this issue! Mine was multi-faceted as well. I had a factory radiator with the lower 6 rows completely plugged with sediment, and at least 8 more partially plugged. It ran hot all the time, and extended runs out of overdrive saw near overheating temperatures. A replacement radiator saw much improvement, but still not quite right. Turns out my fan clutch was weak as well.
I agree with your assessment about the radiator size. People have said it has plenty of capacity to do the job but it seems small to me, especially for a 18,000 pound rig. Subsequent years have much larger radiators as well.
Your rig looks to be in beautiful shape, good luck to you!