Do have to complain about the documentation in the gameheader utility. The N64 document is absolutely inaccurate and uninformative.
0000h - 0003h (4 bytes): initial PI domain 1 settings so you can read this piece of hardware
80000000 hardware bit indicator to determine formatting
00F00000 initial Release Duration setting
000F0000 initial Page Size setting
0000FF00 initial Pulse Width setting
000000FF initial Latency setting
0004h - 0007h (4 bytes):
FFFFFFF0 Clock Rate
0008h - 000Bh (4 bytes): Program Counter (PC)
000Ch - 000Fh (4 bytes): Release
0010h - 0013h (4 bytes): CRC
0014h - 0017h (4 bytes): inverse CRC
0018h - 001Fh (8 bytes): Unused
0020h - 0033h (20 bytes): Image name
Padded with 0x00 or spaces (0x20)
0034h - 003Ah (7 bytes): Unused
003Bh - 003Bh (1 byte): Manufacturer ID
003Ch - 003Dh (2 bytes): Cartridge ID
003Eh - 003Eh (1 byte): Country code
003Fh - 003Fh (1 byte): Version code
0040h - 1000h (4032 bytes): Bootstrap
About that first word, the most you can do is check for the 0x80 byte to determine byteswapping. The other PI values can differ depending on the hardware the ROM was encoded on. Examples would be the 64DD IPL and certain development eeproms.
The lower nibble of the clock rate is not read. Otherwise, if nonzero, 3/4 of this is used as the actual clock rate. This does impact speed on console!
The program counter sets the address you boot from and when doing certain kinds of resets. Certain CIC chips will alter this address by a given amount, in the same way these chips affect the CRC calculation.