There is an easier way, isolate the flow and return pipework and remove the ex. rad. Hang the new 5' rad on the wall, you can now reuse one of the existing flow/return tails if this suits (means rad will be off-centre though) or you can simply cut back tail a couple of inches and fit a new elbow to extend pipework to new valve location. This can be done with one or both valves. This is almost certainly how any plumber would approach the job- there is absolutely no need to take up floors or interfere with finished decorations , this is not justified. OK you will have some additional pipework (surface mounted) visible but this will not matter if the work is planned and neatly executed.