ADD AN FPU TO MY MAC?  REPLACE THE 68LC040 WITH A 68040? 
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       All 68020 and 68030 Macs that did not ship with an FPU 
  standard (i.e. the IIsi and LC series) have special slots that can 
  accommodate an FPU card.  All Macs that have the FPUless 680LC40 
  CPU run just fine if that chip is replaced with a full 68040 that 
  includes an FPU.  All PowerPC Macs include an integrated FPU.  There 
  is no way to add only an FPU to a 68000 Mac like an SE though some 
  third party acclerators do include FPU's as well as faster CPU's.