Welcome to the forum!
I made some rough calculations of the system that you would like to build and I came with one of the possible solutions:
Lead ball diameter = 48.62 mm
Lead ball weight = 683 g (about 1.5 pounds)
Pouch weight (arbitrarily) = 20 g
Draw power = 40 kg
Draw Length = 4 m (Stretch ratio = 5)
Active band length = 80 cm
Rubber width = TBG 342 mm (about 2.7 full width TBG sheets which are about 126 mm each)
Rubber weight (of this system) = 173 g
Shooting angle = 45 deg
Forks height = 2.7 m (about 9 foot)
Exit velocity = 39.87 m/s
Exit energy = 542 J
Range (distance traveled) = 150 m
(all under given atmospheric conditions and gravity)
I made software for these calculations (Slingshot Setup Designer) but it does not work readily with such extreme values so I would take these numbers as an estimation.
Another thing to consider is that if your forks are some 9 foot tall, than to stretch your rubber to 4 meters of draw length in a 46 degrees angle I think that you would need to dig a hole in the ground so that you can manipulate the system as described above because under these conditions your pouch would come to some 0.7 meters from the ground, maybe about ones knees and it would be almost impossible to manipulate 40 kg in that position (you would have to bend down).
Still another thing to consider is that 40 kg of draw weight is probably too much for one person (sorry if I am wrong!) and if this is true than more than one person is needed to stretch the rubber. If this is so, then the problem arises of simultaneous release which I think would be very difficult; you should also think of some "handles", that is, how you are going to hold that pouch no matter if one or more persons are involved (some automatic pull and release system is always possible..)
Finally, the most serious problem remains what treefork said above: return to sender (and I add fork hit and hand slap) which might really be disastrous.
I hope this helps,