Yes, I have. Yes, it fits. Yes, there are multiple 390/FE brackets, and I have not tried them all, but should position the alternator at the same position with any of them, in order to align with pulleys.Has anyone put a 3G alternator on a 390 in the high mount position?
Important - I use the short-ear 3G alternator for most things, as it has the same mounting bolt locations as the 1G and 2G it replaces. Other versions (Taurus, F-150, etc) have different spacing, and not recommended. An easy lookup for the parts counter or online shopping is 1995 Ford Mustang - any engine. There are others, but that one always gets the 130A short-ear version.
Any short-ear will do, and the difference then is the clocking of the rear case for wire access. Changing this is simple and there are many online references to do so. They also come in various flavors of voltage regulators, but none with concern, such as "soft-start". Shopping today will find them new from under $100 to the moon, or at the wreckers from about $20 with warranty, and a free test at any auto parts store. I suggest using stock (not 1-wire) for best operation and regulation. Wiring is simple, and if you can do one wire, you can do two, for better results.
At the bottom is a PDF article I wrote about it... OMG gaining on 20 years ago. It includes multiple 'fixes' for stuff you may run across (or not), and I'm adding the basic for-anything wiring image as well. I also have the earlier generator conversion wiring diagrams if anyone needs that. Hope that helps!