Milton,
Where to begin? If you plan to repaint the exterior, it would be best to go down to bare metal as you currently have newer paint over the original factory paint. If you don't go down to bare metal the paint will eventually crack, crawl or peal off. If you do decide to paint the exterior, that would be a good time to remove the entire front clip to paint hood, fenders and inner fenders. It would also be much easier to gain access to painting the engine while in the car. Or remove the engine and detail it on an engine stand. Whether the engine is in the car or on an engine stand, I'd recommend removing the intake manifold, valve covers, oil pan and valley pan cover for stripping, cleaning and repainting. The block can be cleaned per Walt's suggestions. With the front clip off, you can easily remove the exhaust manifolds for cleaning and re-coating. A couple friends of mine used a product from Eastwood with great results.
http://www.eastwood.com/calyx-manifold-coating.html With the front clip off, it's also much easier to clean up and paint other engine components and front suspension parts. I know that your GS could use a new engine wiring harness. All this would really make the engine compartment look like new.
I had my bumpers re-plated by Northstar plating in Brainard, Mn, which is now owned by Keystone Automotive. Great work at a reasonable price. Drop them off at a local Keystone Automotive store and they will take it from there.
To remove the GS specific exterior trim from the sail panel, you may need to remove the interior sail panels to gain access to the screws that hold it on. Hopefully, you can do this without destroying the headliner material on the sail panels as they are glued in place.
Well that's my 2 cents worth. Winter in coming!
John