Automatic or manual?

Another thing to consider is if you engine does have a true 550 hp in a very heavy car like yours, the standard 4 speed stuff will not cut it in the long run especially if you want to run at the track a few times with slicks or dot DR. I would want a blow proof bell housing, a high end clutch, stronger drive shaft with big joints, do you have a Dana? The 8.75 will at least need axles to last a little while. Plus your closer to needing the 18 spline the 23 will not live forever if your actually going to drive it like the 550HP engine was built for.