i've used similar epoxies designed for this particular use, and i can honestly say that i've never had one that didn't still sweat fluid out from underneath the epoxy. i've used on both iron and aluminum trans cases (as well as magnesium), and also iron/alu cylinder heads with cracked coolant jackets, even engine blocks too. The best of them worked very well for months, and then all of a sudden would develop their own cracks, or worse- the edges would come away/raise from the substrate, and even break off into tiny chips at the ends because its smeared on very thin at those points.
the only time i can say i successfully used an epoxy of that kind for what it was intended was to repair a wollowed keyway on a crankshaft that should still be operating today (i think) and prevented me from replacing the customers motor outright- it wasn't worth it.
I did actually use JB weld one time to restore metal on a gouge that someone created when doing a previous cam seal repair on my old VG30, and all I had to do was smear some on the aluminum gouge with a sliver of an old plastic/credit card, and then hit it with 600 grit paper to smooth it down to accept a new seal, but the seal is what's holding in the fluid in this instance- no sweating of fluid there.