I have a Current USA Retrofit Kit, which said 24" when I"ve bought it. Just measured it and it's a little less than 24" long COMPLETELY, which all cables and endcaps. So 48" should fit a 48" long hood, but I'm not 100% sure with T8, because mine is compact flourescent. Maybe somebody else here can measure their bulbs for you too.
Previously: 2x 10gal tank, 45gal aquascaped, 70gal aquascaped
Currently: 16gal FW, 40W CF, ADA Aquasoil Amazonia II/Tahitan Moon Sand, PH 6.5, kH2, gH 4