Ben, on my 1986 Syncro, I've had  times when the engine coolant
temperature went way up (to the point of the LED flashing). But this
was only on VERY steep grades, running high in 2nd gear. If towing
and/or running the AC it could happen even sooner (and in 3rd gear.)

On my 1991 Syncro Westy, long grades would definitely causing
overheating problems. Monarch Pass (US Route 50 in Colorado over the
continental divide) is a very long grade, and was too steep for 3rd
gear, so I was in 2nd gear at 4000 rpms. About halfway up, the
temperature started climbing (and no turn offs), the LED started
blinking just before I found a turn off. And I was spewing coolant
and all. But not from the radiator. I'm sure the altitude didn't help
(and my radiator fan was running full blast.)

In Kansas (same Westy) heading west, 4th gear and trying to maintain
speed with the AC going, was sure to get the temps rising.

When Smallcar put the SVX engine in they did replace the radiator. I
might have too much cooling capability now (at least in the winter).