Biggest issue is that old rads are imperial sizes and new ones aew metric which means that the pipes are to fare away to connect with rad. I've used these in the past. They are cutable to size and screw into rad where valve goes.
Yes, hang radiator first then cut pipes to fit. If you've not taken a radiator out before be aware the "water" inside can be black. After draining the system there will be, depending how old the radiator is, black gunk in the bottom. So when you disconnect the rad from the valve make sure you've got a container underneath to catch any drops and turn the rad upside down (making sure bleed screws are tight) asap before you carry it out of the house.