OK, now I got a problem and need some help.
I was excited in my earlier post when my start-up went great,
But when I went for my "20 min initial run-in"...it was a different story.

'53 full pressure 235
First run-up after overhaul and it went like this...

Started up, running great first 6 or 7 min.,
Temp was coming up, thermostat is a 160.
Plan was to continue filling coolant when thermostat
opened.
170-175 I started getting nervous.
180-185, 190 I shut it down 7-8 min into first run-up.
It continued to climb some after shut down.

Here's what I found,
Radiator flows great with hoses disconnected.
I didn't suspect the water pump as I had it on my 216 with no issues.
I don't suspect the belt, fan turns when cranking, crazy as it sounds I didn't remember it during run- up.
Pulled the thermostat, it was open,
set it aside and it cooled and it closed.
Opened the drain plug/valve at the rear of block (driver side)...nothing came out. WTF
Completely removed the plug/valve, filled the neck at the thermostat (with thermostat removed) and still...nothing came out at the plug/valve hole.
I distinctly remember comparing head gaskets (old and new) and they were a perfect match.
The block was cleaned in a tank at machine shop during rebuild.

I'm at a loss as what to do.
Thanks, Jim