I drive a KW with a Cat 3176 set at 330 and a 9 speed.  With a loaded grain trailer it is hurting for HP.  If you are pulling on the level it will work ok.  The terminal I deliver to the most is along the river and very level almost the whole trip and you really don't notice the lower HP.  I do have one place I haul to with a good hill going into town and it will be down to 25 mph.  Most of the time this one pulls a machinery trailer and with most loads it has enough HP.  The only thing I normally haul that makes it really work is a big SP chopper.

If looking for something to pull a full load I would look for at least 425 with a 13 speed.  Less will get the job done but it will be slower.  The one nice thing about the small block engines is they do weigh a little less.  With the grain trailer my empty weight is about 25,500.  It could be lower but the truck has a wet kit for the machinery trailer and a headache rack.

Gunner