Heater Bypass Nipple Seized

I finally got back to the car.

A friend had a ¾ pipe tap. I tapped the hole with the manifold in place but had to go deeper than usual would with a pipe tap in order to get into good metal.
I installed a brass ¾ NPT to barb fitting but it screws in almost all the way due to the hole being a little oversize now. I put on some high temp thread seal and tightened in the fitting. I hope this works.
Now on to the next problem…..

Thanks to all for your suggestions.