Cars of a certain age can be fully registered here as Left Hand Drive, so you're right in thinking that 1969 Mustangs can be on the road here in both configurations without any legal problems. As far as RHD being more marketable and desirable, it goes without saying that it all depends on the individual of course, but to make sweeping generalisations I'd say that most people here who use their cars as weekend hobby cars keep them LHD, and most people who use them as a daily driver prefer a conversion. And with these cars being what they are more of them are used as hobby cars rather than daily drivers. As for myself, I'm in the market for a 65/66 fastback to use as a daily driver (I already have a weekend hobby car) and although I haven't totally made my mind up yet I'm heavily leaning towards a conversion. So there is a market here, just probably not a big one.