The engine blocks used in 65/66 Mustangs (also the 64 1/2s, which are titled as 65s) were actually very few. You had two different 6 cyl motors, the U code 170 C.I.D. and the T code 200 C.I.D. You also had the F code 260 C.I.D. V8 along with the D, C, A, and K code motors which were various configurations of the 289 C.I.D. V8. The D code motors had 5 bolts holding the bell housing onto the block, the C and A code motors had 6 bolts holding the bell housing to the block, and the K codes could have been either 5 or 6 bolt, depending on the production date of the car. The 5 bolt blocks were phased out in aug/sept of 64 in preparation for the regular 65 production year, so if your K code had a scheduled build date of oct of 64, it would have had a 6 bolt block. If it had a scheduled production date of may of 64, it would have been a 5 bolt. Even though it was not a clean break on a certain day, if your K code was built before aug 10 of 64, it was probably a 5 bolt and after that was probably a 6 bolt.
The 260 block should have a casting number of C4OE-6015 followed by either a -B, D, or E
The D code 289 should have a casting number of either C4OE-6015 -C or -F
The A code or C code 289 should have a casting number of C5AE-6015-E, C5OE-6015-A, or C6AE-6015-C
All 6 bolt K codes have a casting number of C5AE-6015-E
5 bolt K codes will have a casting number of C4OE-6015-C, or -F. There was also a 5 bolt K code block with the casting number of C3OE-6015-B, but that was not originally used in Mustangs. Also, if the motor has a 3 or 4 digit number stamped on a pad on the block over by where the clutch Z-bar is attached to the block, that motor came out of a Fairlane. The K code motors that left the factory with a 65/66 Mustang wrapped around them had the vin stamped on the block on the passenger side, right above the top of the oil pan, towards the front. Sometimes it is stamped so poorly that it is very difficult to see, let alone read, but it is there.
Right above the casting number will be a date code. How to interpret that is explained in the section I called, that's right, 'Date Codes'. The date code on the motor should be consistent with the scheduled production date of the car. Ford did not put a motor built in aug of 65 in a car that was completed in oct of 64, nor did they just sit on a motor for a year waiting for just the right car to put it in.
This one is very simple. If you pull the valve covers off, you will see on the top surface of the head either the number 260, 289 or 302. The 65/66 Mustangs did NOT come with 302 heads. Ford did not start building 302s until the 68 model year, so, if the heads on your car have the number 302 cast into them, they are definitely not the original heads for your car. There will also be date code on the top of the head. Again, the date code on the head should be a little bit prior to the scheduled build date of the car.
The K code heads were different from the others. They had screw-in rocker studs, instead of the press-in studs the other 289 heads had. They also had little pockets that the valve springs sat down into to keep them from scooting around on you at high rpms. They will also have either the number 19, 20, or 21 cast into the front outboard corner of the driver's side head and the rear outboard corner of the passenger's side head. The head shown in the pictures is a K code head.
The intake manifold will have a casting number on it right in front of the carburetor.
The 4bbl intake will have the casting number of C4OE-9424-H
The 2bbl intake will have the casting number that starts out with C4, C5, or C6, then either an O or an A, followed by an E, which is followed by 9424, then dash something. There is a date code at like 2 O' Clock from the distributor, which, again, should be consistent with the production date of your car. There were a whole bunch of different casting numbers for 2bbl intakes, so who could really say which is which. As long as the date code matches up, that might be the original intake.