I got a reply from Gougeon Bros:

Hi Steve,

I believe G/flex Epoxy would hold up to the chemicals and conditions in an
automotive cooling system without a problem. Our Standard 105 Resin/205/206
hardener epoxy has worked for this as well but G/flex has higher heat
resistance so I would probably go with it instead. G/flex 655 is the
thickened version of G/flex. It would be nice to use it if the pitting is
on vertical surfaces or where gravity is not in your favor. Otherwise
G/flex 650 (the liquid version) would work as well. The aluminum boat
repair kit includes the liquid version (G/flex 650) as well as a powder
thickener in case you need to make thicker epoxy.

Be sure to scratch the aluminum (a wire brush would probably be best for
getting into the pits) just before applying the epoxy.

Best regards,

Thomas Pawlak | Technical Service

Gougeon Brothers, Inc.