Almost forgot this one.
A redneck idea method that I came up with few years back and method I've used with good results is take a air chisel and get a blunt nose chisel or just take a 3/8 bolt about 6 inches long and use it as the blunt air punch, connect the air chisel to a air pressure regulator and set the air regulator at real low pressure and hold the trigger down on the chisel and start SLOWLY turning up the air pressure and when it gets to around usually 25 to 30 psi the punch when held tight against a object will cause the object, rusty seized bolt or stud to start singing and vibrating like a wood bee is inside and it will almost sting your finger when held to the metal or bolt. Sometime you can actually see the rust crawling out from a large rusty stud. This breaks loose the crud and rust inside around the bolt/screw threads. Then for bolts always use a 6 point wrench instead of a 12 point that rounds off the head and watch the bolt or stud carefully and when you can wiggle it little bit use care and keep wiggling gently back and forth instead of just trying to hoss it out in one shot.
This really works great around old manifold bolts and studs.
I've used this method on rusty manifold bolts before they twisted off and also on studs.
Also is a good try for steel bolts that are galled into aluminum.
