As I understand it - the problem of engine fit on the 737 is due to the height of the undercarriage which Boeing have resisted lengthening due to a major redesign of the wing and fuselage centre section. This was coupled with not wanting to raise the height of the fuselage so that the baggage loaders did not need specialist loading equipment - a key consideration for short turn round LCCs. However that hasn’t stopped easyJet from operating the A320!