Hi All
I agree with Harry about the Fenton manifold, it really is the fix for exhaust manifold problems. Having said that I'm reassembling my HUP with the original style manifold just because the HUP is otherwise original. I think I can get away with it because the HUP doesn't have to work the engine as hard as the bigger trucks.
On my C60S I ran the stock manifold for years having to replace the manifold gaskets about once a year because they would develop leaks and twice I replaced the manifolds with better condition ie flat and not eroded ones. Reason for failure I'm pretty sure was the newer gas with just runs hotter and the big trucks tend to have to throttle positions on the floor and all the way up.
The Fenton two piece manifold was the solution I put it on and that was the last time I had to touch it for nearly 10 years. When I swap up to the 261 engine I used the same Fenton manifolds so far no exhaust leak.
I would add a comment from a CCKW expert he suspects that the manifold gasket problem is three fold: one the new gaskets are not made of asbestos, two the new gas just burns hotter, three the replacement mufflers cause to much back pressure. His fix new muffler punch extra holes inside the muffler, little more noise but less back pressure.
Just to add to the info here is another view of the pipe.
Cheers Phil