Im using a 3SGTE engine (turbo) that will be good for about 170-180kw (230 - 245hp).
To make sure the IRS at the back will handle that, I really supported everything in a big way - double upper and lower bracket carrier beams jointed together to prevent twist, some diagonals etc. The wishbones are 25dia x 2mm tubing.
At the end of the day the rear end may look like a Chines bamboo scaffold with all the tubes, but Im sure will get the power down on the tarmac, and the little extra weight at the back wont hurt either
Here are some pics.