A tank assembly and a stiffening assembly include first and second inserts and an elongate coupler. The inserts comprise a base having flange, a head, and a neck. The elongate body has first and second couplers at respective first and second ends of the body, each coupler including a pocket sized to receive the head of a respective one of the inserts. The related method include forming a fuel tank such that the tank wall carries at least a first and second insert, locating a rigid connector within the tank interior aligned with the first and second inserts, the connector including a first coupler and a second coupler, and coupling at least a portion of the first insert with the first coupler and at least a portion of the second insert with the second coupler to limit movement of the two wall portions relative to each other.