They used the same fuselage for everything from the 707 through to the 757 (obviously not the 747), but the wing config and/or engine placement were all different; the larger engines on the 757 meant longer landing gear too. They didn't get it wrong till the Max.
With the Max am I correct in saying the gear is the same as the classic, but the geared engines created the issues due to the pylons being different? Maybe TD Racer could comments, as he usually has great insight into those sort of things?