Jason;
If the header leak is causing the paint to discolor you should not drive it until repaired. The bolt is 3/8x16 and can be replaced with the engine left in place, sometimes from under the car. It sounds like the gasket should be replaced also. I have changed a few big block engines in Uniques cars so I know of what I speak. I find a standard hex head header bolt is eazier to put in than a allen headed header bolt. Just take care to starte it stright, in tight spots they cross thread eazy. Good luck
Poorboy
PS. Make sure to check all the bolts on the other side also. Header bolts are bad to work loose.