Hello there! I can't answer in great detail, however I can say that throughout my Slackware installation, there are many references to settings intended to be used on 64 bit machines, and there are also some options within the kernel which are specific to 64 bit architectures as well, such as 64-bit memory usage, encryption algorythms for 64 bit machines, and likely other things.
Slackware may or may not be classified as either for 32 bit OR 64 bit, however it will certainly run on both types just fine, and has capability for some 64 bit specific capabilities.
The i686-class question is definitely a YES, to put it simply.
And as time goes on, it is likely there will be more and more 64-bit specific options to choose from; as to whether the designer of Slack will decide to produce a "Specifically 64-bit version", my thoughts are NO, but I'm definitely not the final word on that.
As for your kernel compilation options, you basically must research your computer, the motherboard, and the hardware, and if you want to compile a kernel, then you can use the info you learn about the computer, and tailor the kernel to your system.
Every kernel can be compiled a multitude of ways, and for a new(er) system, you would want to use a 2.6 kernel almost certainly, to get the most performance out of your computer.
I hope this gives you a little bit more info about what you would like to do.
Best wishes, from Canada to Greece.