I now only use this product recommended by a life-long professional engine-only rebuilder:

gasgacinch
I "glue"/seal the gasket to the sheet metal part, and also use a little engine oil (or grease) on the other side of the gasket.

http://gasgacinch.com/

Two essential precautions/actions:
- make sure sheet metal surfaces are straight and smooth and contoured where appropriate
- do not over-tighten the sheet metal screws

I have had no leaks over many years.