[mzs]

Doesn't look too bad particually the match tagged version last:

  MyType= (Scalar4, Real4, NullTerminatedStringC);
  
  MyUntaggedRecType=
      RECORD
      CASE MyType OF
      Scalar4: (longC:  ARRAY[1..150] OF longint);
      Real4:   (floatC: ARRAY[1..150] OF real);
      END;
  
  MyTaggedRecType=
      RECORD
      CASE tag: MyType OF
      Scalar4: (longC:  ARRAY[1..150] OF longint);
      Real4:   (floatC: ARRAY[1..150] OF real);
      END;
  
  ...
  
  { set all to 0.0 without running through the MC68881 }
  FOR j := 1 TO 150 DO
      longC[j]:= 0;
  
  ...
  
  CASE tag OF
      Scalar4: percentReal = longC[1];
      floatC:  percentReal = floatC[1]*100;
  ELSE
      percentReal = 0.0/0.0;

edit: don't have a pascal compiler handy, but that's the idea