not that i have done it but hard, not really. massive time consuming job, yes.
Pretty much you are looking at not just engine loom but the whole lot - can prob leave some of it, lighting circuits probably can be left but thats about it. So looking at completely stripping the loom from the 60 (so pretty much remove the interior including dash then pulling out the whole loom), do same in the GT but taking care to label all the wires so you know where they go, taking pics is always a good idea so you can see how things are routed etc.
So yeah, not really a hard thing to do but hugely annoying and time consuming.