Almost any diesel performance shop should carry them...there are 13 in PA listed on the south bend website
And you can build an auto to hold the power you make. Many of the parts used in the diesel auto trans will also work in a gas auto.
I had a BB 727 built using a lot of diesel trans parts that would hold a twin turbo'd 12v ctd. Just have to find someone to build it, or do it yourself.
It can be done, if you want to (and it didn't cost anymore than a regular performance rebuild really). The biggest cost on my rebuild was the CRT full manual reverse valve body & HP pump