some info on a 440

Confused. What is overheating the tranny or the motor? The original truck rad is twice the rad they put in cars. Still could need a cleaning but as was said- it will do the job. Is it an old big block 426 bored to 440 or is it a 440 out of a 68 charger. Or are you saying you have a 68 charger 426 hemi bored 60 over?
The 426 pick ups were a mid 60s option and very few were built. I remember 383 as the big motor in the 67 p/u and most of those were the 3/4 ton.
As everyone says- probably you are low on fluid- maybe wrong dipstick, or bands are out of adj or fried.