Hotrod68

3.55's should work well for you. POSI won't adversely effect handling. Whoever said that does not understand what a POSI does.
Whether to piece together a 9 inch or buy new probably depends more on your own knowledge than anything else. These days you're not likely to find any 9 inch rears very cheap used or not. It's easy to get the wrong used one as well unless it comes straight from the same year car as you are going to put it in and then you'll have to put money into freshening it up. If you have your heart set on a 9 inch (based on your HP numbers you don't need to upgrade from an 8 inch) I'd recommend you buy a new one from a reputable source like perogie or currie and save the hassles and stress of trying to find a fit (unless of course you really know what you're doing in terms of Mustang rear ends).
All that said, I think if you just put a new 8 inch diff with 3.55's you'll be set.