I'm a keen DIY mechanic with my vehicles. But exhaust manifolds are a ball-ache.
It's often one of those "commit or quit" jobs.
If you have MAP Gas or Oxy set, angle grinder, Dremel, anti-seize compound, tap & die set, comprehensive set of spanners etc then
you can tackle it yourself - with patience.
But expect to break, remove & replace at least one stud or munt the threads on another.
Some will take a while to move the nut back and forth to slowly work it off the stud. Patience is beneficial - if a nut doesn't want to unwind in one go, don't rush it.
Start soaking all the nuts & studs with penetrant at least a day before (or even longer).
Another product that has helped me is Loctite Freeze/Release spray. It's expensive, but it works in conjunction with MAP Gas or Oxy.
Some nuts will take a few heat & freeze cycles.
There's some good tips & tricks on YouTube for using a welder to remove stubborn nuts and studs too. Especially after you have either snapped one or given up & cut it with a grinder or hacksaw.
And if there's any chance that you're keeping the truck long-term use lots of high temp anti-seize when re-assembling.