Don't know about the Poston's converter, but can tell you from experience I would NOT recommend a HUGHES converter. Hughes was recommended by DRW here in San Diego, 10 months later converter took a dump wiped out the tranny, DRW pretty much washed their hands of it said tough.
I have spoken to many and there are only really a few choices of converters, 9X11 from Bruce, or you can go to the converter shop, where Bruce get his stuff, ask for Rusty, you can get a precision single or multi disk, or a modified stock converter. I would not fool around with a poston's convereter, they are not a converter shop, only a retail outlet for other vendors product. I only say that based on my experience with the Hughes unit, not becuase I have heard bad things about Poston's unit.
Good luck, choose wisely, tranny rebuild are not cheap.