I could not tell when the last 289 Mangusta was built. I have access to a very early car and will attempt a more in depth look on my next visit. You are one of the privileged who has seen the comparable engine VINs and the other option well, more expensive, would be to acquire the build sheets. We know officially that no 289s were produced by Ford after July 1968, motors may have been put in cars out of sequence who knows at DeT. Some Mustang guys claim they have 289 blocks with 302 rotating assemblies but again who knows. The J-code motor is just as good. If you have a good engine builder he can pull a few simple tricks without going wild (8MA1266 & 8MA998). In the case of the Mangusta (and Deauville & Longchamp) the intake is the trackable component , there are other indicators without opening the motor but the intake carries the VIN.