Sandblasting will not hurt the brake lines, it will prematurely wear out the rubber lines, it will get into every nook and cranny, it will be a real mess to do laying on your back, it will not remove any undercoating that you may have, it will embed the sand into the undercoating you may have. If possible get it up on a lift and use and use a wire brush by hand, a wire brush on a drill, a putty knife, and a power washer to get it as clean as possible. Then get some POR-15 and paint the underneath. The only other way is a frame off job and I really don't think you want to get into that. One other thing you may want to look into...A couple of guys told me about it that went to the Cars & Parts swap meet...is this contraption that bolts to your hubs and is like a huge loop. You take off the tires, bolt this contraption to the hubs, drain the oil, gas, tranny fluid, and radiator and you can roll the car upside down. It is different than a rotisserie. If I can find it on the net I'll let you know. Weimer knows what I'm talking about because he's seen it and maybe when he gets on here he'll remember the name.
Kevin