The last time I messed with the hot rod it had a stripped hole for an exhaust stud. Since these holes are wet what's the best way to fix it?
Will a heli coil provide a watertight seal under radiator pressure or should I look at doing something else?

it should if you use sealer or maybe even some j/b on it. That or you can re-tap it to the next size up metric. I'd drill it, tap it, run the sealer all over the coil, set the coil, let it set-up then run your stud/header bolt in as usual. I hope it's off the car.

Should be no problem with a heli coil.

I will be different but I would tap it out to the next over size and use a step stud. That way there is only one potential leak spot to seal rather than 2.

One of the guys over on BBD used a threaded insert that he got from Grangers. He said that he didnt like the heli coils. With the threaded insert he used an epoxy. There was never an issue with coolant leakage.

We had our max wedge heads repaired with time-serts. Threads can be sealed and top part of insert is tapered and seals well to the block. Much better than heli-coils.
Time sert