Update on pics what you need and what you don't need this is to help all that have this problem.
Needing the right parts for the right Engine.Some engines are way different.No bagging on the newbies.But this is a crash course on what to look for at the swap meets or Ebay or Craigslist.
I have added all 3 exhaust manifolds 2 are right for your truck the other is for the car or Corvette.
One thing about exhaust manifolds You are not forces to use gaskets You can have them planned meaning you put them on a big sanding belt and have them evened.Then bolt them to your heads.
most people thing No you can't do that.GM been doing it ever sense they had exhaust manifolds.
reason for this is exhaust manifolds flex heads do not unless you lay them down flat.
But on the side of the head where the exhaust manifold bolts to never seems to have a problem.
Now back to the lesson.There are 2 types of exhaust manifolds one is a straight down shot
The other is the Curve reason for this was GM moved the motor forward a few inches so that's why we have to types years to look for is 1963-67 had the straight some late 67 had the curve and 1968-72 had the curve.
Now the 2 brackets that make this whole system work are still sold today LMC and
www.truckandcarshop.com on the top bracket that holds the top of the alt it might need a little modifying but noting a vise won't take care of.
Now your trying to figure out why GM had 2 different type of heads.
That was how GM learned what word and what didn't.Plus Gm never looked at heads as a place to put brackets later they learned it was a good place.
So i have added all the pic's to help you in your search to put your truck back together right with the right brakets