just find the best shop around and have them rebuild it, tell them you want a billet converter and request it be built for 500hp that pretty much tells them it needs to be stout.
just lie and say I plan on putting a supercharger on it and I want the transmission to be able to handle that.
then you have a local warranty and nothing to worry about.
a good shop will know what needs to be done, if you do not hear things like extra welds, extra clutch's, non shrinking check balls, etc then just walk away and find another shop.
there is a term called rebuilt which is bare minimum work required to be called "rebuilt" you don't want the bare minimum, you want it done right.
if you buy from somewhere online or out of town, ya you might have a warranty but you will have to jump thru there hoops if anything crops up later