Going to Hollowtech will not work unless your 73mm bottom bracket shell is threaded for Italian, and even that is a safety concern. Problem is, English Hollowtech spindles are sized for 68mm bottom bracket shells, so fitting one to a 73 means you'll lose 5mm of overlap between the splines of the left crank arm and the splines of the spindle. This is asking for trouble. If the left crank arm doesn't eventually fall off, its splines will at least become permanently ruined.

Look for larger chainrings for your MTB crank. You should at least be able to get a 48-38-28 setup, but be careful about the inside ring hitting the chainstay.