FIC's Taiwan site is the one for answers.
FIC Taiwan
JE4330 or JE4331 will work for the 503+ (also needed if you run WinME with any K6)
Get it from a link at the above site.
If you need more advice, the following newsgroup will help.
alt.comp.periphs.mainboard.fic
I just bought one of these to replace a K6-III at 448 (112x4), in a FIC 503A.
448MHz was the most I could get out of the K6-III.
To get the same performance of the K6-III at 448, I had to run the K6-2+ at 550Mhz.
But, I was easily able to hit 600, which made the K6-2+ faster.
Also ran it at 112x5.5, or 616MHz, but had occasional errors, so backed it down to 600MHz.
At these speeds, with this CPU, temp is a non-issue. CPU runs about 2 to 7 degrees more than case temp, even in hot Florida room.