8" is going to be a tough thing to do with any ordinary home handy man type tools. Even renting a gas concrete saw is only going to give you a about 4" or 5" depth. Meaning you will have to cut from both sides. You might want to look at hiring that part out.
Do you plan on turning the crawl space into a full high basement? If so you need to worry about the foundation. Lowing a basement floor means you need to extend the found walls down, and pour new footings and under pin it all together.
Do you plan on turning the crawl space into a full high basement? If so you need to worry about the foundation. Lowing a basement floor means you need to extend the found walls down, and pour new footings and under pin it all together.