It sounds like your clutch is either out of adjustment or worn out. Was the clutch set up properly when the tractor was sold? They do NOT come from the factory with the clutch adjusted properly – it has to be done prior to putting the tractor in service or the life will be short. Also, riding the clutch or using it to govern ground speed will wear it out in no time at all. Tractor clutches are meant to be all in or all out, no in between. Ground speed is controlled by the gears and, to some extent, the throttle.
Yes, your 354 should easily lift that brush hog. A 6' brush hog is the biggest one it will handle in heavy bush, but that is a factor of PTO horsepower. The 3-point hitch will lift over a thousand pounds so it can lift an eight foot brush hog, but you don't have the PTO power to run one that big.