function setFieldsByWidth(   i,n,FWS,start,copyd0) {
  # Licensed under GPL Peter S Tillier, 2003
  # NB corrupts $0
  copyd0 = $0                             # make copy of $0 to work on

  n = split(FIELDWIDTHS,FWS)

  if (n == 1) {
    print "Warning: FIELDWIDTHS contains only one field width." > "/dev/stderr"
    print "Attempting to continue." > "/dev/stderr"
  }

  start = 1
  for (i=1; i <= n; i++) {
    $i = substr(copyd0,start,FWS[i])
    start = start + FWS[i]
  }
}

#Note that the "/dev/stderr" entries in some lines have wrapped.

#I then call setFieldsByWidth() in my main awk code as follows:

BEGIN {
  #FIELDWIDTHS="1 3 8 8 5 9 1 9" # for example
  OFS=" "
}
FNR==NR {
  FIELDWIDTHS=(FIELDWIDTHS)?FIELDWIDTHS FS $0: $0
  next
}
!/^[  ]*$/ {
  saveDollarZero = $0 # if you want it later
  setFieldsByWidth()
  # now we can manipulate $0, NF and $1 .. $NF as we wish
  print $0 OFS
  next
}