I'm using bolt in subs from competition engineering, but I welded them in. For the rear, your can jack it up and support it by placing jack stands under the axle. This way the chassis is loaded the same as if the tires were on the ground. For the front you can jack it up and place some wood blocks under the tires, that should give you enough room to weld, but it won't be comfortable by a long shot. Welding while lying on your back under car is not a pleasurable experience, be sure to wear some thick gloves, a helmet that covers your whole face, and maybe try to cover your body with a thick blanket or something. Those sparks will fall down on you and they can hurt. Other than that, just be sure to clean all undercoating, paint, grease, galvanized coating, etc from the surfaces you plan to weld. A sanding wheel on a angle grinder works well. Take your time and do it right, you don't want to have to cut them back off.