Re: Exhaust manifolds or headers?
I have a 1990 Class "A" with a 460 and I had the passenger side manifold warp on my unit. I removed the manifold and had it machined to true. Then I installed studs into the heads, renewed the manifold gasket and used nuts with thick Belleville washers. I now have over 10,000 miles on them and there is no problem. While I was at it, I did the same to the driver's side.
Two more things that I did is that when I shut down the engine, I give it a little cool down time before I shut off the engine. The other thing that I did was, while the manifolds were off, I sandblasted them and coated them with Never-seize. They look almost new today. This is an old trick that I used with aircraft engines. Let the manifolds cook after you do this in a well ventilated area, as they will stink until cooked. But no more rusty look in my wheel wells.
I did buy a set of Headers, but due to my Oshkosh frame, I would had to have moved all the brake lines, master cylinder, cruise control module and several electric looms to allow them to be cool during running with headers. Not worth the effort.