Yep. Exactly this. It's unfortunate because, as I had posted in another thread, I had hoped to replace a 60 or 66MHz 601 with a >100MHz 601v or 604 but it's not possible because of the voltage differences to the 601v and the reassigned pins on the 604. A 6100 with a 120MHz 604 would be pretty cool but sadly is impossible.
Are the pinouts different between the 601 and the 601v or is it just that the 601v expects 3.3v where the original 601 used 5V on the power pins? Oh, and I/O as well. 3.3v vs. 5V. Interesting to look at the Power Computing Power120 PPC601 surrounded by level shifters. At least I think I remember correctly that those are level shifters. It's actually been a decade or more since I looked at it...
Excellent table of chip packages and compatibilites. Very nice.
The PowerPC 7410 can apparently be had in 600-700MHz varieties, and it's these that you'll need to use if you want a B&W G3 or Yikes! G4 on a full 100MHz memory bus.
For some reason the later models with PowerPC 745x variants use FPGAs to interface to the memory bus, and these FPGAs can't operate faster than 66MHz, thus the requirement to drop your bus speed. Oddly, the same 800-1000MHz 745x chips don't appear to require FPGAs in the older beige Macs.
The difference between the 7410 and the 745x chips is the use of full-speed L2 cache and the inclusion of L3 caches on the 745x where the 7410 has only half-speed (at most) L2 cache. You'll still get a good performance boost out of the 745x-based upgrades even with the memory bus speed drop.
The best 100MHz bus-based chips for these are the 750GX, which has 1MB of full-speed L2 cache in addition to core speeds up to 1.2GHz. These are oddly hard to find though, even though they should have been less expensive than equivalent G4 upgrades. Possibly because, without AltiVec, they struggle with some of the pretty stuff under 10.4 Tiger and can't be used at all with 10.5 Leopard.