It's been a long time since I've thought about those things, but IIRC the only actual issue with the Spectra is physical clearance. In other words, it'll work fine, but you have to cut a hole in the back of the casing in order to fit any kind of heatsink on the CPU. Which isn't necessarily a bad idea anyway, since those things are pretty limited on cooling... unless you're looking to keep the thing looking original for whatever reason.
You can also modify those things to provide the proper voltages for K6-2 and K6-III (and even K6-2+ and III+ chips) directly from the motherboard, though it involved some soldering and was a bit of a messy hack if I recall. I've also heard that some later revisions of the motherboard didn't need to be modified at all, just some jumper/switch settings changed, but I've never encountered one of those so I'm not sure on that. Might be worth checking out, though, in case you have one of said revisions...