Thanks for the ideas! Someone on another forum has suggested a much easier method though...

I feel a bit silly now.

EDIT: Found another post suggesting that piggybacking the VIC-II's and toggling chip select won't work. Hmm.

There are transistor ICs that you cam use that'll give you a whole array of them in a small package but there will inevitably be a lot of soldering.  Another option is to go with very small components. I always have a load of surface mount digital transistors (MMUN2211 work great as switch replacements). Some people get scared of surface mount but it's actually not that hard to learn and doing a large number in a row will actually go pretty quickly once you the the hang of it.

In any case, you'll want to tie the bases of every single transistor together so that they each activate simultaneously and as a safety feature you might want to set up a simple circuit that cuts the C64 power if you attempt to switch modes - a DPCO switch that cuts power in the center position just to be safe when switching modes, maybe? I'm not entirely sure what the best solution would be in that sense.

I've read that converting a C64 between NTSC and PAL requires swapping the VIC-II chip and crystal, and connecting a jumper. Surprisingly simple. Even better, the VIC-II in my C64C is socketed. It would be cool though to have both NTSC and PAL selectable with a switch, on a board that plugs into the C64's VIC-II socket. Unfortunately, 40PDT switches (because the VIC-II has 40 pins) don't seem to exist, and if they did they'd be a pain to wire up. I'd like to design a functional equivalent. I don't know that much about electronics, just enough to follow other people's instructions for modding, but I've never designed anything from scratch before. So maybe my ideas will sound naive...

Transistors can work like switches right? What if the line coming from each socket pin on the motherboard splits into a Y, with one direction going to the NTSC VIC-II and the other direction going to the PAL VIC-II. Each chip would have a transistor per pin. Have an SPDT switch to apply voltage to the bases of every transistor, with one throw connected to the NTSC chip's transistors, and the other throw connected to the PAL chip's transistors. Except... I was reading about the VIC-II and it seems like some of the pins are both input and output. Is that correct? Transistors only allow current to flow one way. I read that triacs are similar to transistors but are designed for AC currents and allow flow both ways. Maybe triacs could be used instead.

Would this even work?
Even if it did, is there a better way that doesn't involve soldering 80 transistors or triacs? Some kind of IC?