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Theoretically, you would need to magnify the image by 1.6X. In practice you will probably, especially at the loner end of a zoom, a little less. One way to tackle this is to use a 1.5X or 1.7X tc/extender. 1.4X is probaly to little, and 2.0X very makes IQ too little to even consider. You would need a 3rd party tc, like a Tamron, Tokina, Kenko or Sigma, as Canon extenders won't work with anything shorter than a 135L.
As others noted, you will not only magnify the focal lenght, but also any lens errors and aberrations, probably downgrading the lens quality by 1 or 2 grades, while making it a slower lens too. At 1.4X you lose 1 f-stop, ar 1.7X 1 1/2 stops, and at 2.0X 2 whole stops.
HTH, kind regards, Wim