have you considered an sti 6spd with a rwd spool? they hold 500+whp on awd i would imagine that you would be just fine with only rwd loads on it. also you won't need any adapter plate made.
i understand you want an automatic transmission though. if you are firmly set on that, make some measurments and take those to a machine shop they should be able to CNC a piece for you. you can make sheet metal templates or something to draw measurements from.