Here is what is behind the manifold stud hole in block.
Since hole is off center best get someone to install cast iron plug and use milling machine to relocate the hole for stud.
This is another reason to measure depth of hole before drilling.
There is very little "wiggle room" here.
The best way to remove broken studs is to weld a washer to stud then weld nut on the washer like mentioned on Vince Falter's website or use milling machine.
Impact wrench to remove studs is a very bad idea also unless done properly.
Permatex #2. It does not get hard and will stop the seepage. I find many blocks that have the head studs drilled or rusted into the water jacket and this works unbelievably well. Does not harden to the point that the stud can not be removed.