Water pump plug is stuck

[Dino]

You know those 5 minute jobs that turn into hours? Yeah didn't miss that... 

I want to remove this plug to stick a temp sensor in its place but as you can see, it hasn't gone well. I cut the paint around the plug, and sprayed some PB Blaster in there that I let sit overnight. Today I stuck a 3/8 ratchet in there but it wasn't the tightest of fits so I couldn't put all my weight behind it. I removed the hoses, so I could have access from above, and figured the impact would make short work of it. And it did make short work of reaming out the 3/8 square...
What else can I do to get this thing out, preferably without ruining more paint?

[Kern Dog]

I wouldn't be concerned about the paint. I'd weld a lug nut to the top of that plug and crank it out with a ratchet. NO impact! The impact will just rattle the edges of the fastener and round them off.
Repainting is a small price to pay. I see no other way to remove that plug other than drilling it out and chiseling the remaining bits. You'd obviously want to to remove it to do that since you don't want metal bits floating around in your cooling system.

[b5blue]

Start with 1/8 drill and work up till you can work an EZ out in there and back that sucker out. 
Use a Shop Vac wile drilling.

[70 sublime]

I just finished filling in that hole with a plug on my water pump housing
Do you not have the small hole on the front side of the housing for the temp sensor ?

[Dino]

Yeah I'll weld a nut on there. That's probably the best way.
I do have the hole in the front that holds the stock sensor. This one is to hold the EFI sensor.

[b5blue]

I wish I had a welder!

[Dino]

I wish I had a welder!

I'm very happy with my Everlast 140 i-mig

[Kern Dog]

How can a car guy NOT have a welder?
EZ outs snap off far more times than they ever remove broken bolts, then you're stuck.

[BrianShaughnessy]

Unbolt and remove the housing.

If you're low buck low tech,  find a yellow map gas canister for a propane torch and heat the F outta it.

Pound a good 3/8 extension into what's left there and use a breaker bar.

Repaint and replace.

Optional:   Throw out cast iron housing and replace with aluminum housing.