Slant 6 leaking from water pump bolt.

I have a 63 Dart that I am finishing off a restoration of and ran into an issue. When I got the car it had the core plugs out and new ones in a box to replace them (they were corroded through). Thus, it had no coolant when I got it. I removed the engine and went over the engine with all new gaskets and new timing gear and chain, as well as new water pump. Also had the valves lapped. Never did a lot as the engine ran good before and is supposed to be lower mileage. Here's where the issue comes in.

When I replaced the water pump the bolt on the lower side of the rad hose connection (the one that goes into the water jacket) broke. I was able to drill and extract it. I noticed when I replaced the bolt it seemed sloppy. When I filled it with water (didn't use coolant as I'm in a heated garage and wanted to check for leaks and also flush it one more time after testing the cooling system) I found water leaking from that bolt. I did apply a little sealant to it but It won't tighten up. It feels like it will strip if I pull on it.

I am looking at a couple options to repair it and am looking for opinions/other suggestions.

First option is to drill/tap it to accept a 7/16 bolt. Not sure though if the 3/8 hole is too close in size to properly do this and get a good strong thread that wont leak (with thread sealant of course).

Second is to drill it out to accept a 3/8'-16 x 1/2"-14 E-Z Lok thread insert. Not a fan of a Helicoil as it's in a water jacket and never had lots of luck with those. Concern here is if there's enough material on that "ear" on the block to allow me to drill it up to a 1'2" threaded hole without breaking off.

I was also considering trying the 7/16" bolt repair and if that doesn't work use the insert. I was reading and if I'm correct, the tap outer diameter for a 7/16" bolt is 0.4375" while the drill used for the insert is 29/64" or 0.4531". Again, still a little concerned about that tap thickness.

Also, if I was to try the 7/16" tapped repair would I be able to accomplish this without removing the water pump? One less step and less parts required would be nice.

Any suggestions or past experience would be appreciated.

This is the insert I'm considering:

3/8-16 x 1/2-13 E-Z LOK Threaded Insert for Metal | Buy a 3/8-16 x 1/2-13 Carbon Steel Threaded Insert - E-Z LOK