That would not be adviseable, even if you could cut the brick without it cracking.
Suggest taking the whole brick out (+ the mortar below) and bed your padstone on new 1:3 mortar on the two bricks below. If your padstone is 140 mm high (?) fill in the gap between the top of the padstone and underside of the beam with some offcuts of steel plate - your beam supplier might have some small scrap pieces.
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