I fit an LRP Z.28 big block engine in my matrix recently. I had to mill the chassis with my dremel tool in some areas for the engine to go low enough for the stock engine mounts to reach the chassis plate. I didn't have to go through the bottom, just milled about 1-2mm in the rear.

However, After trying this engine out in the car, I found it to be way too much power for me.

Hope this helps.

Zac