Early Hi-Po heads have casting numbers C30E or C40E-B. They had 1.67 intake valves and 1.45 exhaust valves.
In mid 1964 casting numbers C50E-A and C5AE-E were introduced. They had 1.78 and 1.45 intake/exhaust valves.
In addition a C7ZE casting number was introduced in 1967.
Later service replacement heads were C8ZE-B.
Hi-Po heads had a little higher compression ratings and also smaller additional "damper springs installed inside the larger springs - but these may be lacking on incorrectly restored heads.
Apart from the casting numbers Hi-Po heads also has cast in spring seats and screw-in rocker arm studs.
Hope this helps you - info is taken from the "The 289 High Performance Mustang" book by Tony Gregory.