I don't know if it will work on a big block, but the only way I have ever been able to get the driver's side header out of my 69 340 barracuda was to put it way up on jackstands, loosen both motor mounts, and (with a floor jack and a block of wood under the oil pan) jack up the engine as high as you can. You will eventually hit the firewall or tranny tunnel pinch weld. It also helps to jack the driver's side up higher than the other side. Then, if you hold your mouth just right and say the magic word, the header can be dropped out the bottom.
