Make your own brackets. It wont be that hard. Buy some plate steel and go to town with some metal working tools. If you need drive shaft work, check out Drive Shaft Services on Cobb parkway/41 in marietta. They do all the work for the offroaders and alot of the guys running big motors in little cars.
Beat the **** out of your drive shaft tunnel because its gonna be alot bigger. How is your rear end? Why not throw an solid axle under there while your at it to make it wont blow?
You can get a cheap 10 Bolt chevy or ford 8.8 for nothing and it will hold the power you need it to.
I dont really see the little IRS diff holding up to 400 hp and off the line torque.