No idea how this works on a B20, but when my '79 240 had similar issues it did end up being the radiator. The key to me is the fact that when it starts hitting its peaks, it could easily be brought down by turning on the heater.

The slowly rising temp gauge started during a bonsai run from Tulsa, OK to Madison, WI and back in 36 hours. This was a B21F with a BW55 and we cruised at 70+MPH most of the time. Any time the grade would steepened even a bit the temp would climb to about 3/4. Turning on the heater, no fan required with the flow-through the older cars had, would pull it back down in a short 10 minutes. The Husky did not like it much since it was only about 50F outside the rest of us did not mind.

When we got back, I replaced the thermostat and water pump. No change on the next highway trip. While the radiator was out for the water pump install I took it to a radiator shop (old Blackstone? brass radiator) and they tanked it and pressure tested it but said it likely needed a re-core. So when things did not get better, I installed my spare aluminum and plastic radiator...problem solved!

Mike