If its 'I' shaped as you say, whack a few wooden noggins into the I section nice and tight and then just screw into them for the sides, and a liberal amount of dri-wall adhesive for the underneath chief
Nog the steel out then measure the depth, width , depth score your plaster board one side and just wrap it round the steel no need to worry about fixing the soffit once its beaded it aint going nowhere.
Lucius
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