The jack supplied just does not seem all that safe to me for doing what you say you are going to do. Floorjacks are not that expensive ($30-$50-and up) and you don't have to get the superduper 5 ton type. A nice 2 -2 1/2 ton, preferably with a big dish, should do nicely. After all, its your brain that is going to be under all of that steel, and it will squish rather easyly under that kind of weight.
-Larry