If you use a CASV Vltor forearm, it basically clamps the the pic rail on top of the upper, and is supported by the stock barrel nut.
If you are going to truly put on a free float fore end, then you will have to remove the barrel (or at the very least the barrel nut). Removing the gas tube can be very difficult if you have fired the rifle a lot. Carbon will build up making the removal difficult. Removing the front sight is also a good way. Bottom line is that you have to get the gas tube out of the reciever somehow. I use either the reciever block or barrel clamps in a 4" vice. That way things don't move around when loosening and tightening things up.