ARP makes some very nice exh manifold studs in their stainless 300 alloy (if memory serves). They're both strong & don't have corrosion issues. Don't know if they make them in the right size for the 6.5?
The bolts/studs that mount the exh manifolds to the heads are a unique thread shape. Don't know if the corresponding thread in the heads is unique? Would seem logical the unique thread is to keep the bolts from loosening while at the relatively low torque spec & resulting clamp load used to hold the manifold sealed to the head. Prolly low clamp load to allow some expansion/contraction movement btwn head & manifold as things heat & cool.
Another thing worth remembering w/ many exh system studs, is that the nut is often some type of locknut (often copper/brass, or some alloy of that color). So they're somewhat designed to grab onto the stud - to avoid exh leak warranty claims. Also some OEM applications used some locktight type thread sealer. So a little torch heat to release that may be in order. Some stock engines are assembled w/ locktite on the studs in the heads. Don't know if this was done on the Cummins heads? If so, again a little torch heat may be in order to release the thread sealer. Probably want to search factory service manual, etc., to confirm if they used threadsealer & if they suggest heat to release it.
For the manifold to cross-over studs, I used reg steel replacement studs, & stainless steel nuts. This avoids some of the tendency of the stainless stud/bolt combo to gall on themselves. Obviously the stud can still corrode, but it doesn't "grow" together like the stock steel stud/nut.