Pentium 3 is a huge bottleneck for the 6K. No point bothering with a 6K if you are going to run one on that combination.
The 6K has drivers made all the way up to Windows 10 now unofficially of course. Windows 98SE / Windows 2000 is recommended for the best performance with a V5.
PCI Sata cards will give nothing but issues unless you buy a promise card such as this one below. The Silicone image/marvel chipset cards have big issues with bus mastering/high speeds on Win98 which cause the mouse cursor to lock up when moving data. It can be resolved with a bios flash/the correct drivers.
The best and most painless solution is below. These have all the correct drivers available & are not based on the chips I mentioned above.
As for running a Pentium 3..
Find yourself a Epox 8K3A or 8K3A+ with the KT333 chipset, AthlonXP 2700+ / 2800+ / 3000+ / or 3200+ in 333FSB flavour. It is the most stable combination and best performer.
Best bios settings are listed in these forums for those boards, and what drivers to use.
If you want to build something period correct for the year 2000 that would be KT133 based Athlon or a OR840 with Dual Pentium 3s