If you are intending to use the same exhaust manifold on your build, don't get too angry with the fixings holding the down pipie to manifold , they are almost certainly going to snap, leaving you to drill the broken studs from the manifold
Here's a couple of ideas.
Pull the manifold assembly to one side, tie it back out of the way & sort it once you have removed the engine.
Secondly, chop the manifold off by cutting through the downpipe, leaving a few inches of the downpipe for you to play with when fabricating your Roadsters exhaust.
Very unlikely that you will need the Sierras downpipe in its current form .
Now, I can't remember if the exhaust is mounted to the manifold using nuts & studs or bolts.
Eiter way, heat will help .... Heat very hot and then quench with cold water ( This will destroy the rust bond ).
If nuts & studs are used, grind one side of the nut away, taking great care not to hit the threads with the grinder .... Heat of grinding should break the bond, if not, once almost down to the thread, spread the nut a little using a chizel.
This job will be much easier with the manifold removed from the car & the rusty threads can be cleaned up with a die
