Nix on the 8"x18" base idea. While that is about the size of the grinder base, to side mount the motor like using Rob's mounting plate, you need 18.5"x18"5" as measured on my KMGs. If you want to rear mount the motor, you'd need a deeper platform, but I wouldn't take that approach as it would make the assembly very deep. You could under mount it, but with a belt change machine, that would turn into a back breaker after a while. Be careful of making the base too small, these are heavy machines that you will end up pushing on. You certainly don't want to build a top heavy assembly and have it end up falling on your foot!
I've got one bench mounted and the other one on a free standing pedestal. The Pedestal is about 16"x20" 1/2 steel base with a 6"x8" steel I beam welded to it with a 3/8 top plate. I used Rob's mounting plate for both and recommend it highly. One of the things about a KMG is the smoothness, so the more rigidity and mass you can build into the base within reason , the better.