Big block Mopar are all studs. Heat the nuts with a torch and they will usually come off. Do not try and pull the studs! They will break off and they go to water, resulting in a coolant leak. The bolts to the exhaust pipes are good because you just break them off as the studs on a GM do and then knock out the bolt halves. If the exhaust at manifold has been changed usually some effing gorilla tightened them and the flanges are often bent.
Now, let me open my gas can up here and throw some on the header fire. Headers always work better, period. They work at low rpms they work at high rpms. The gas mpg's they return in this age of high gas prices will pay for them in a few years of solid driving, not going to a local car show every 3 weeks for 3-4 months in a year.
However header prices are completely out of control. There are no cheap headers for a C body and thus no source for used swap meet headers. So with that info just keep the manifolds and a nice dual exhaust system. It is easier and the 2 pipes will help move air through the engine better resulting in better performance and mpg. Don't fall for the HP manifold trap, they do not work any better than the log manifolds on a stock low performance car. They still dump into a common chamber at random or opposite angles. This provides no scavenging for the next cylinder to exhaust and thus no advantage.