Small block radiator size/cooling issues
318/340 A/C radiator, shroud, A/C fan and fan clutch should fix you right up.
Slant 6 rads are made for 225 cubes.
And a seperate tranny cooler helps, esp. with a high stall converter.
Check this one out. 2nd to last picture.
http://www.forabodiesonly.com/mopar/showthread.php?t=93176