It's slightly more complicated than just getting an 8600 or a 9600. Both of these models were produced with different logic boards in separate revisions of the model, one accommodating the 604e processor and the other accommodating the 604ev "Mach V" processor (which is what you have). As has been said, you need a machine with the "Kansas" logic board, and not one of the earlier, 604e-specific, 8600s or 9600s.
Now these "Mach V" machines are not especially rare, so what you might do if/when you get one is to experiment with the resistors on the spare card, and keep a processor card stock.
I run an ASIP web/fileserver on an 8600/300 604ev and it is very snappy and able. I noticed a big difference, especially for fileserving, between it and a 9500/200 604e that I had otherwise configured identically earlier along life's way. The 604ev really is an excellent processor, which gave the early G3s a run for their money in many processor-intensive tasks (e.g., better FPU scores).