Hi again.
On the 289 thing, the 'D' code motor was a 289 4 bbl, but it was a 5-bolt, meaning that the bellhousing was held to the block with only five bolts, instead of six, like a 65 model car. Also, the K code motor became available in the Mustangs in June of 64, but again, these were 5-bolt motors. The K codes became 6-bolt motors around the end of July of 64 and started making their way into Mustangs in mid aug. The earliest K code that I've ever seen that still had it's original motor and was a 6-bolt had a scheduled production date of August 12th of 64, but the heads were both mid July, and the block itself was late July.
There is a long list of things about the 64 1/2 cars that is different from what was on the 65s, but the big things would be that the 64 1/2s had generators instead of alternators, 5-bolt V8s instead of 6-bolt and the Dearborn built cars will have a sequential number lower than 250000, and the San Jose cars will have a sequential number lower than 125000. There was a big blank spot in the sequential numbers used lower than that. There also weren't any Metuchan built 64 1/2s, nor were there any 64 1/2 fastbacks.
As far as the parts supply thing goes, there wasn't any clear cut-off. Most of the stuff was just used until they ran out. That 6-bolt, alternator charged K code that I mentioned had a 64 1/2 hood, a steering gear box tag with the code HCC-AX-1, which is a 64 1/2. The 65 K codes had HCC-AX. It had a bunch of stuff that is considered 64 1/2, but the sequential number was slightly higher than 125000 ( it was a San Jose car) and was a 6-bolt, alternator car, so it was a 65. Some of the 64 1/2 stuff was used all the way up into October of 64, which is well into the 65 model year. Ford wasn't in the business of making parts to throw away.
how do you tell if it is a 64 1/2 or a 65? 
