The Big Input Toploaders only came behind big blocks and have a shorter input shaft, which means to install one of these behind a small block requires a longer pilot bushing to support it.  This handy bushing does that, you can see the difference between a stock Ford one and this one in the pictures.  It also has a 'landing' to stop from it being driven in too far.  These made from 'oilite' material intended for pilot bushings.  Also work for most 6 cylinders in Broncos and F100s.  Discontinued.