I have the same problem. I stopped by a radiator shop to see about getting a new tank top. The main guy said it would run about ninety bucks (California money), that's parts and labor to remove the old one and install the new one. I didn't ask but I'm sure that's if I bring the radiator in.
We took a shake down cruise in the "new" 81 300SD last month and went up into your Country. Its all I can think about. We did the "Selkirk loop", (northeastern Washington, Idaho panhandle and into B.C. through Nelson). It's my kind of country!
I just remembered that Longston posted a quick temporary fix for the broken neck problem. I looked for it but couldn't find it . What it amounts to is...before you tighten the hose clamp, run a piece of tiewire under the hose clamp then under the spring clamp in front of the radiator then join the two loose ends of the wire and twist until tight. I only did one wire but if I remember right his picture showed a second wire tied to another point.
Good luck and lets hear how things turn out.