The PowerPC G4 7400 is just a PowerPC G3 750 with a SIMD unit stuck on its linage goes back to the 603 not the 604e
on the subject of the 1Ghz Pismos, there are pictures of said upgrade online and its interesting as its literally a stock Pismo CPU card with the G3 PPC750L removed and a small interposer PCB soldered on with a G3 750FX/GX stuck on top of that
they even left the backside L2 cache on although of course that now does not do anything.
it would be fun to reverse engineer the small interposer board, and then make something like a 1Ghz iMac/iBook G3 or such
iv also heard there was a Prototype 900Mhz 7447A G4 Pismo upgrade card I would love to see pictures of that and see how it was put together.
I actually have a small gaggle of PPC750GX/GL chips and I hope to solder one onto a 750FX iBook G3 at some point and see if it works

(the 750FX and 750GX are pin compatible, the 750FL and 750GL are just low power variants)
(I have 2 750GX and 3 750GLs there the chips next to the 3 MPC740s)