Here's something that may be helpful. It's a link to the Hayden website that explains the difference between the various types of fan clutches. Thermal vs. Non-thermal. Standard duty vs heavy duty vs severe duty: http://www.haydenauto.com/upload/HaydenA..._TS_6980012.pdf

For those of us with '62 - '65 B bodies we don't have much to choose from as the Hayden 2765 is the only one short enough to fit. Other Mopars with more clearance may have more choices.

Here's a link to the Hayden catalog that includes all the dimensions of the various clutches:

http://www.haydenauto.com/upload/HaydenAuto/Documents/Cat_Hayden/2013_HaydenCatalog_FINAL.pdf