In my opinion, checking or replacing the 'stat is one of the FIRST things you should do. Quality parts aren't what they once were. Remove the 'stat, go to the grocery store, and get a "candy thermometer." Put both in a pan of water, and observe WHEN it opens and how far. If there is ANY doubt, buy a new one and put BOTH of them in the pan.

TIMING:

There is NO REASON to fight questionable timing marks, it will only take about 1/2 hour to put them 'right.'

Get or make a positive stop like this:



Take a plug, hacksaw off the rear crimp. Drive the guts out and cut off the ground. Weld a nut on the back, or thread through the middle for a bolt. Play with bolt length as needed.

UNHOOK THE BATTERY!!

Remove no one plug, feel around for clearance, and screw in your stop

With a wrench, rotate the engine gently until it stops.

Make an ACCURATE pencil mark on the wheel under the TDC timing mark

Rotate the engine the opposite way until it stops, make a SECOND pencil mark under TDC.

Your true TDC is halfway between the two marks. Now just figure the "degrees / inch" around the wheel, and you can mark the wheel from there clear up to 40-50 degrees, or just buy the proper size timing tape. Be sure to check the tape!!

FAN

What fan are you running, are you sure it's not installed backwards, and is the clutch 'good' if it's that type?

Pulley sizes, belt slippage?

Does the car overhead on the highway?

Wind the car up to "high cruise" RPM and hold it a few seconds, and observe the lower hose. Any signs of collapse?

Does the heater put out "good?" If not you may have a pump problem

Extremely lean carb conditions may worsen overheating problems

How about plugged exhaust, is there ANY chance of something there? Running stock manifolds? Is the heat riser butterfly opening?