Is 300 enough, no. Even the guy that has 600 still looks for an upgrade to 700:) I have a 99 discovery on a freighliner chasiss with and ISB 275 HP, @ 650 ft.lbs. torque, and a 6 speed allison probable pretty close to the same set up you are looking at. I tow a slighty modded jeep that is just over 5,000 lbs. and I have no complaints with power. Do I WANT to upgrade power of course and probable will with the Bank's kit. Do I NEED to upgrade no. I can tow the Grapevine at 50 mph with the jeep in tow all day long and with the exhaust brake I can go down the other side without touching the service brakes. I am a big fan of getting the most engine and trans you can but at some point there is a trade off with budget, floorplan, and power. I settled with a smaller engine for an interior and floorplan. Somehow going up the grade at 10 MPH slower doesn't seem to matter once I get to camp:C