If you have no sign of leaks. It may be that your coolant system is need of a serious proper bleed. Forgive me if I didn't see it, but I dont think you mentioned that you bled the system. If could be air forcing coolant out while driving.

Im not a pro but I don't think not machining the heads would cause this to happen. If it were that I'd think you'd see it leaking with decent inspection. I did not machine my heads and I'm 15,000 miles in with no issue.(Knock on wood).

Bleed the engine when it's cold. Unsnap the reservoir and elevate it 8 inches, or above the upper t coolant hose. Unscrew cap, then unscrew the bleeder screw. You should hear some gurgles. You can do this now or you can also do it again when you change the t-stat. But regardless you must do it after you install the 180 t-stat to ensure no air in the system. You may need to do it a few times.

 

We've had guys do the job without machining, then have it leak, and when they take it apart and to the machine shop they find out they were out of spec by some number. While it is not needed to always machine, it is a good idea to have it checked. The flatness spec is .002 inch, about 1/2 thickness of your printer paper.

Without coolant loss, HGs may not be part of the problem.

 

1) Replace the Fan clutch. It is shot. Even if the fan still has resistance - the clutch is probably 1/2 gone. Replaced mine with Hayden Heavy Duty. Smartest thing I ever did.

2) Replace the T-Stat with the 180 degree. I see you are already doing that.

3) probably replace the radiator too.

This is the trifecta and your horse will win if you replace all three of these.

You will never have cooling problem again.

Well, if you are getting exhaust gasses in there then all this does not matter.
You are back to square one.

 

Update.

I pressure tested the system.

There's a little hose that is attached to your intake and goes to the tbs heater. When I get to about 8psi the hose starts spewing running down the valley pan gasket onto the back of the engine and down the bellhousing. Weird because it didnt drip before. The pavement was dry...but the temp went up.

Now i have a small leak...just have to find it. I will also bleed the system the way you suggested.

I added the 180 stat and am running mostly water with a half gallon of coolant. My temps are 194 while driving. While at a stop, they hit 210 but the fan kicks on and takes the temp back down.

Once I am confident about the head job I'll change to green coolant and change the ratio.

I didn't shave the heads because....
I have shaved heads in the past and they gave me issues with the former of the intake.
I did take a straight edge and light, feeler gauge and checked them that way.
There is also a certain way to REMOVE heads so they don't distort as well.