Slant Six overheat at cruise speed, especially going uphill

have a look...same issue...
http://www.forabodiesonly.com/mopar/showthread.php?t=43837

A quick test would be to put a hose in the top of the radiator at full blast with the lower rad hose off...if the radiator can't drain faster than the hose can fill it..it's not flowing well enough...