1. do a compression test to be sure you are not blowing exhaust into the water jacket, despite what you have done to the head, etc. Be sure to re-torque the head several times; once is NOT enough
2. Double, triple check your timing. A late motor will run hot as can be. Timing screws up more Barners than I could ever have imagined. Despite that you have another A that runs fine. If you have a timing lite, put'er on there just for kicks and giggles. Review Marco's pics of how the rotor should be; it can't hurt.
http://www.abarnyard.com/workshop/timing.htm
3. be certain the overflow pipe is up in the rad. as far (high) as it can go; put a large roofing nail in the pipe per wild Bill Williamson (search here) or use a short length of fuel line hose to extend the tube up as high as you can get it; the A water pump is
known to wanna pump all its precious water out the overflow
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