Overheating issues on a 383 Polara

Infrared thermometers are freakin awesome for troubleshooting clogged rads.
Check the temp at:
The inlet neck.
The outlet neck.
Various places on the front of the rad - ( look for "cold spots" ) will tell you if some of the tubes are clogged. The biggest variation in temp will be on the first cycle of the t-stat

From you last post it sounds like it's overheating just sitting at idle?

Take a look at this...
I know it's from a ( GULP, prepares to duck and run ) "honda" site, but the concepts are the same. (just replace the "fan not coming on" with "bad fan clutch" :)
http://www.tegger.com/hondafaq/overheating/index.html