Answer: You could certainly plant the taller bare root privets and trim them back. Generally, a new privet hedge is trimmed back short repeatedly during the first few years to encourage it to grow dense at the base. So, this would not be a hardship for the plants at all.
Alternatively I suppose you could root tip cuttings from your existing plants and grow your own, but they would take a lot longer to match your existing hedge than using the three footers would.
The other thought I have is depending on why you have to replace such a long run of hedge, if there was a lighting/drainage/disease/pest problem that could affect your replacements, you might want to look into using something other than privet rather than replanting with the same thing. I hope this helps.
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