While you're backing those bolts out, if one tightens back up as you remove it, take a moment and run it back in one thread, and then start backing it out again. If the threads are binding up on rust, running it back in a little smooths things out. You also need a feel for whether the bolt is turning or if it's stretching, and that's a hard "feel" to teach over the internet.
If you are referring to the exhaust manifold bolts there is a tool that is placed between the front exhaust port and the middle exhaust port on the manifold that spreads the manifold open slightly to take the side load off from the bolts to ease their removal. GM put out a service bulletin on this years ago when these trucks first came out.
Snapon used to offer the tool.
There are some engine families where the exhaust manifold bolt holes in the cylinder head are open-ended, you can spray penetrating oil on the back side, directly onto the threaded part of the bolt. You stand a chance of having the oil actually wick into the threads and do some good.
On a SBC, none of the holes are open on the back side. The penetrating oil would have to get between the underside of the bolt head and the iron manifold. This is practically impossible. Then the oil has to travel down the un-threaded shank of the bolt to even reach the threads.
Penetrating oil will make a lovely but useless puddle on the ground.
I would spray the bolts with PB Blaster at least once a day until you are ready to make the attempt. It's very heat tolerant if you choose to heat them up with a torch. I got a 26 year old o2 sensor red hot before the PB Blast boiled off. It's evil smelling stuff but it works well.
I put shorty headers on my 135,000 mile 1995 C2500 New Yuck state vehicle in February. Every bolt unscrewed with no heat, no lube, no problems.
Maybe I'm just lucky, but I've never broken a 3/8 exhaust bolt upon removal. Not like the tiny, weak 8mm Ford or Chevy LS.