Help finding part

What size thread does your exhaust manifold have? I believe some had 5/16-18 coarse thread and the others were 3/8-16. Don't think they were fine thread but could have been.
It's been a while. Easy enough to check. I think even if you have the small threaded exhaust and an intake with a fat hole you can just use a washer under the nut.
Or if your exhaust is the bigger thread and the intake is the smaller hole the intake can be drilled/reamed out for the fatter stud. Plenty of thickness there.
Just whatever you do DON'T do like the PO of my truck did... They snapped the stud and drilled from the bottom of the exhaust manifold to try to get to.... Actually they did get to it and thru it, and put a bolt up from the bottom thru both manifolds with a nut on top.
Luckily they didn't get this conglomeration reassembled to the engine, I went to the junkyard and got another exhaust manifold and started that part of the project over... .just to see if that engine would run. If you do have the exhaust manifold rethreaded for the 3/8 stud remember it's a blind hole.
When I put mine together I ordered #316 stainless fasteners from McMaster Carr.
There's a diesel shop nearby that has a laser doohickey that cuts out broken bolts without touching the parent part, I wound up taking my exhaust manifold to in order to have all the old stuff layered out. All 5 bolts on this manifold were busted. Now that I'm thinking about it there were 3 sizes on these manifolds. The 2 outer bolts that hold them together are 5/16 thread. The stud you were talking about is 3/8" and the 2 that hold the exhaust pipe flange to the manifold are 7/16". These are thread sizes not wrench head sizes.
Prices may vary for this, but I feel like I overpaid, but I already had a bunch of time in that exhaust manifold in porting, matching the outlet size to the most commonly available flange gasket, I didn't want to throw "my" work away.
This was a different one than the one I got from the junkyard for the original engine.
The owner of the shop (is the only one that operates that laser machine) and is a buddy of mine, but I made the mistake of handing it off to the grandson who is a gold digger.... When I had the same job done on my sons truck they were much more reasonable.... Somehow I think it got "made up for and then some" when they have to do the exact same job for ""dear old Dad ".... But that's a story for another day.