Thanks, Kestas, I'll keep an eye on it.
There was actually more than one leak, but the shop pumped it up to a higher pressure than found in normal service to test -- 20 psi. At normal operating pressure, it seems tight.
The rad itself is a nice all-metal piece of work. No plastic necks on this thing! It's a shame that the PO let it get stuffed up.
And actually, I found two other radiator shops' tags brazed on to the rad when I pulled it out. Apparently the PO paid to have the rad cleaned out several times before, without ever inquiring into the root causes of the overheating issue. People. Sheesh.