Hello Nick,
I believe I posted this option to you when you first introduced yourself on this site. I have a friend who is a long time Ford guy,he worked for Ford for many years. He has a Brand new NOS(new old stock),250 Six cylinder sitting on a pallet that he would like to sell.I believe his asking price is substantially less than what you could buy the rebuild parts for your current (worn)engine,not to mention,labor,machine work prices(not cheap). Perhaps you may want to price parts and labor on an engine rebuild and get back to me? Remember that whatever you end up with when you buy a core or a rebuilt engine you are still getting a "Used" Engine. To put it into more modern terms my friends engine is a "Crate motor" It is brand new,never been run. Personally I would have pounced on it for one of my cars,but 65-66 cars only came with 170/200's and 289's.So that's what I run in my cars,Call me a stickler.
P.S. Mustangs were not available with a v6 Until 1974.If you look at the arrangement of the cylinders in your current engine you will see that they are "inline" or "straight",so your engine is technically an "Inline six" or a "straight six".