Personally, I wouldn't try to run a 6ft bush-hog behind the 3040. Not only do you have barely enough horsepower to do just very light grass, but a 6ft'er is going to be very physically large and heavy to handle with a tractor of that size. If you get on much of a slope and have to raise the hog to turn or go over something, you'll have very little control, even with the weight of the FEL.
I'm running a 6ft'er on my JD 2030 (65 hp) and MUCH larger frame size, and it's all that tractor wants, but I have to cut some pretty high and tough weeds and brush periodically.
On my LS R4020, I'm running a 5ft Modern Ag Products Super Sunshine Cutter which is a medium weight bush-hog, and it's perfect for that tractor. It'll chop up most anything that that tractor will ride down. I'm often in weeds and brush that is from 3ft to 6ft tall and tall fescue along the fence lines and ditches, and it takes running right at full PTO rpm (about 2400 engine rpm) to keep the power up and not bog down.
On the 3040, I'd run a light to medium duty 5ft'er, that's around 375 to 500 lbs or a heavier duty 4ft'er that's no more than 550 to 600 lbs.