change rear end ratio or change transmission

I have a 98 379 with a 550 emodel , 18spd, and 370 rears.
hauling cattle and hay. my problem is im topped out at 72 mph and cant pull a hill in 18th this truck has all limits taken off , turned to 550hp, pulls great in everything but the high hole. my question is would i be better changing to 355 or 336 rears or change to a 13 over tranny and leave the 370s? i dont need to run 120mph, lol i would just like to run the speed limit in some states and be able to hit a hill hard enough that im not running 50mph at the top.
thanks

 

ive got plenty of hp i guess what im wanting is a higher top end ie; set the cruise on the flats and be able to run 75-85. i know it wont pull a hill that fast. just tired of cruising at 68 . it will run 68-70 loaded at 90k but at 70mph im pulling 1650rpm with tall 22.5 tires