Someone on another site suggested that a heat shield would help under the carb to prevent boiling.Is the heat shield a 1/16" plate that sticks out from under the carb 2" or so all around the carb?

Yes, those plates really work.  Make sure you use a gasket above and below, and retighten after a week or two.  I had a radical small block that would start to run rough at a long light.  I just added the plate, and it would idle forever.

Larry

Carburator spacers come in many thicknesses. They really isolate the heat. Here is a sampling of some of them:  http://www.jegs.com/webapp/wcs/stores/servlet/category_10001_10002_10289_-1_

I read about those, too.