Like stated above, definitely invest in some jack stands, you can get a pair on Amazon for around $25 shipped! Really not something to mess around with, especially when the solution is so affordable. You should never get under a car like that. I don't want to scare you but if one of those jacks ever failed there's no guarantee that those bricks/slabs wouldn't slide out from each other.
Anyway, back to the bolts. Use a 1/2" socket on the bolts with something long for leverage whether it's a breaker bar or a torque wrench, something to give you some extra torque. You can also try putting the closed side of an open end wrench around the bolt and hitting it with a rubber mallet/dead blow hammer. The sudden force should break the bolts loose.
Obviously an impact wrench is your best bet but I'm assuming you probably don't have one or else you would probably already have the bolts out. Heating them with a torch is also good advice and will help.