Found this pic on the 'net. It doesn't look too bad, (or I've seen worse), would probably look better without the porthole windows, or if they had utilised an Australian AP6 or VC Valiant utility rear window styling.
Found this Dart Utility conversion picture on the 'net. Looks OK except the porthole windows look a bit weird. If you could combine this idea with a roof/b-pillar/rear windscreen concept similar to an Australian AP5/AP6/VC Valiant utility would look better.
A couple of pics of an Australian VC Valiant utility to give you an idea of the roofline, plus someone in Victoria (Australia) has done an S-Series Valiant with AP5/AP6/VC Valiant roof. This is what you should aim for if you're serious about it. Good luck with it, look forward to seeing it.