FWIW ...As well as the above unit, Temro do another for the 1.5" welch plug, 400W, 120V, 3100084, also cheap. Alternatively there is a unit designed to go in the lower radiator hose, 375W, 120V, 1-1/4" hose, 3200002, around USD50. Several hose sizes available.
2023 Phillips and Temro Product & Application Guide
2023 Phillips and Temro Product & Application Guide