Yes, the FPU runs at 50Mhz on B1230-IV cards.
You need a 68882 PGA FPU rated at 50Mhz. Vesalia used to sell (perhaps still do) 40Mhz 68882 FPUs that were tested OK at 50Mhz. Perhaps you could give them a try.
EDIT: I'm not sure about the 33Mhz PGA 68882. That would be a large overclock. B1230-IV is hot as it is, and this would probably make it even hotter and probably the FPU would die pretty quickly.
