I suspect a 1/2" NPT tap, but I am guessing. You would need to measure the inside diameter of the hole and see what tap size would work best.
Whatever size you use, don't get to crazy with it, remember, pipe taps are tapered. Tap and screw a bushing into the end of the exhaust manifold and then screw the EGT probe into the bushing. You may have to retap the bushing inner treads to match the EGT probe threads?
I would use some JB Weld on the outer threads of the bushing to manifold to make it permanent.

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