Just my 2p mate .....when you cut the metal away ,I'd watch that... a)you dont get carried away cutting too much away b)make sure you radius the corners ,so that there are no square cornered stress points where a split could start.Seen a plasma cutter in use ,but never used one .....personally I'd have a go with the grinder first! But others on here may have different ideas! BTW did you fit the air originally ??