[rockosocko]

I know it might sound goofy, but on a Sunday or weekend, find a warehouse somewhere that has 'ramps' to where you can come in from the side and get that nose pointed up as much as you can. Pop the hood, and the radiator cap and see if you can "burp" any air pockets.
Another thing is that although some people have the original engine, it's NEVER been flushed correctly, and after 50-60yrs, there's a lot of rust and crap down between the cylinders in the block. And that insulates the bottom of the cylinders.

There's a bunch of better flushes out there now. SO doesn't take a really long time to get all that crap out. Even if you can get those two pipe plugs out of the middle bottom of the Chevy blocks. And a bent piece of coat hanger to dig around while the water is being forced through.

And with all these 'import' Thermostats, a couple 1/8" holes drilled around the skirts can't hurt. That way you always have some kind of water flowing around. Even if it sticks shut. (..funny how they never seem to stick OPEN!)