If the threads felt OK I would use Permatex Aviation grade and if there loose I'd go with the Permatex #2. Both available in good parts houses. This will work only if the threads have enough integrity to hold the torque loads themselves.
I would also set the manifold gasket wet so sealing with a bit less torque on the studs is possible. Here I'd use the Av grade.
About how many threads do you think you will have in the block?
Dave Neptune