look at the printing on your cpu's to see what the core voltages are. the ver5 core voltage can be changed down from 2.8 with switches. use the search function on this board to find the info.
the buss speed can be changed by installing switches or soldering jumpers, and with software.
remember - the faster you run the cpu, the more heat you will generate. the cpu will also draw more current to the point the i-o cannot supply enough to run stable.
with external core voltage power, switches and software, and assisted cooling, you could get up to about 600MHz if you are a good technician.
your k6-II-300 will probably run at 200 MHz ok but the -400 may need too much current to run in the i-o without help, even at 200MHz.
if you want to hack your i-o, the easiest way is to get a bios chip and hard drive cable from Badflash. if you want to run windows, see "Two new iopeners, what to do with them" up to 3/18/02 on this board. if you want to run linux or some other o.s., use the search to find drivers for the chipset, display and modem.
if you run into problems, come back and ask.