Heater bypass?

>>So even a half blocked radiator will still cool an engine sufficiently until the engine is put under load.<<

that is exactly how mine acted. It was fine, cool even, until I went from 45 to 65 mph and 5th gear. Then she ran hot,and once it got hot, it didn't really cool until it sat and I started again.
Commuting to work at 45 it was good - the lower edge of the green. On the Interstate it went to the top edge of the green.

Did you price a rad for an Eagle?
Should fit the 660, maybe by swapping brackets side to side or not at all.
90% of AMC's cars take the same basic core and only the tanks and brackets vary 

If no radiator shops around to recore it then get the Eagle radiator. Might be simpler to do that and have a shop swap your brackets onto the existing radiator. That could be a welding shop -- the brackets are "silver soldered" onto the metal tanks (not regular plumbing/electrical solder -- "silver solder" is really soft brazing -- takes higher temp than solder, but less than brazing with brass, and strength wise is in between also).  You CAN do it at home with a hand torch, but get a MAPP gas bottle instead of propane. Propane might work but you have to heat the material for a long period of time, enough that the heat can get down to the tank seam and cause melting. It's not just the temp, but how long it stays hot too.

There used to be rebuild kits for the Ranco thermostatic heater valves used in the 63-65 Ambo/Classic, but I don't think they are available anymore. I've bought aftermarket replacements, but they seem to fail after 3-5 years regular use -- might last longer in a show car that seldom gets used in winter. I replaced mine with a plain old on/off valve. Works better and won't leak like the thermostatic valve will. I used a generic in-line valve in a heater hose (I think it's a Chrysler valve), but there are some that have a firewall bracket (such as a 64-69 American w/232). A 90s Honda valve will work. Anything that fits the hose and will clamp a cable on will work fine.