Just found one at Brothers Chevy Parts, part #HBB0059 looks like it would fit.
They say it fits up to '59. I've never seen on like that on a stock TF truck, all the ones I have and have seen are a retangal inverted boot that hangs below the floor with a metal trim ring held down by (I think) 6 screws.
Maybe someone can confirm it's correct on the AD trucks...
Mike B
