Old World PPCs have a 4MB ROM, 3MB of which is the "mac" or 68k ROM image, and the last 1MB is the PPC code to initialize the hardware and the 68020 emulator to be able to execute the mac ROM.
The mac ROM is more or less an image the same as mac roms have been since the beginning. It has checksum(s) internally that cover what it knows about, but it doesn't know about the PPC ROM. So the mac ROM checksum only covers the first 3MB of an Old World ROM.
Here's is a little utility that shows the PPC checksum information, and how the checksums are calculated, so the entire 4MB ROM can be verified.
http://synack.net/~bbraun/macsrc/ppccksum.c
It also prints out a version string found in the ROMs. Here's sample output from the {6,7,8}100 ROM:
Possible ROM Info found at 30c000
ROM checksum of byte lane 0: 2dcbf0e
ROM checksum of byte lane 1: 3396836
ROM checksum of byte lane 2: 265e5ff
ROM checksum of byte lane 3: 2bda730
ROM checksum of byte lane 4: 2b66423
ROM checksum of byte lane 5: 3964e19
ROM checksum of byte lane 6: 2883f03
ROM checksum of byte lane 7: 2e38f21
ROM checksum: dedc602cfc3b9221
Image Base: ffcf4000
ROM Size: 400000
ROM version: 0
Mac ROM Offset: ffcf4000
Mac ROM Size: 300000
Version string: Boot PDM 601 1.0
Calculated checksum: dedc602cfc3b9221
Lane 0 checksum: 2dcbf0e
Lane 1 checksum: 3396836
Lane 2 checksum: 265e5ff
Lane 3 checksum: 2bda730
Lane 4 checksum: 2b66423
Lane 5 checksum: 3964e19
Lane 6 checksum: 2883f03
Lane 7 checksum: 2e38f21
All checksums match
