Interesting, but if you expect to run OSes on this unmodified, it won't work: Embedded PPC chips only support compatibility with the userland programming model, not the supervisor one, specifically regarding things like the MMU. So you'll need some things to emulate it.
The other way I'd say yes, this way I'm not sure, you'd have to check the developer notes for both models, I recall they changed a pin function somewhere in the line because the ROM wasn't on the board anymore.
Btw, having seen this thread about C22, I just confirmed mine has the + pin of C22 at -5V, which is not really a good idea. It still works but it will probably shorten its lifespan. Not sure what it would do to tantalums either…
I have similar plans but for ADC first… Not even sure I have a machine with this connector anyway.
As for pins, it's possible to find bare pins by 100 instead of rows which won't have the pitch required. Worst case you'd use two layers of PCB to keep them aligned and straight.