X2 on the studs.
You can torque the stud into the manifold tighter than the nut on the stud with no issue.
If you want some very strong studs that you can get locally, then find most any truck dealership, i.e. Mack, Kenworth, etc., and ask for some turbo mount studs and nuts for a mid '90's or older. The older stuff on the Macks and Caterpillars weren't metric. The studs will be course into the manifold and fine thread for the nuts which equates into better clamping value at lower torque value and also will come apart easier in the future.
Lastly,
Never seize is your friend. Don't forget to use it!
