Exhaust manifold/header advice sought

Its a DIY until one of the studs breaks off flush with the engine block. You can hopefully avoid this by spraying the rusty exposed nuts with liquid wrench or such for about a week or so. If you have a grinder you might try to grind the exposed nuts off tto - they pretty much won't be able to be reused anyhow. Once you have the header off look to see if it is cracked or where there is black on the gasket indicating a leak. Before putting it back I'd check to see if the sealing surface is warped (machine shop can check and true it up). If its cracked JB Weld might fix it. Some say you can weld cast iron but I'd look for a used oen. Headers reduce low end power in favor of high rpm power -- so aren't much good if you climb rocks. If they aren't stainless or ceramic coated they normally don't last long --- especially around salt. I wouldn't worry about engine braking -- disc brakes are cheaper than clutches.

Headers will have no change on engine braking in the mountains or elsewhere. On long downgrades with a heavily loaded rig, the engine must be used as the primary speed retarding force. If the brakes overheat you crash. You want to downshift early and pick a gear low enough that it just about holds you at the speed you feel safe for your rig on that road. Brake moderately hard to get your speed below that safe speed, then get off the brakes and let them cool. When you're back up to your safe limit, brake again, then get off the brakes again.

Headers are tuned for the rpms at which you want the most power boost. The shorter the header primary tubes the higher the rpms where the boost comes in. Shorty, block-hugger headers are just about useful only for engine decoration (and replacing cracked manifolds). Long tube (about 36"), equal length tubes, not too big in diameter provides a low and mid-range boost and doesn't hurt top end.