try this first..
assemble everything and pressurize the crankcase.. use an air hose and a regulator and push about 20 PSI of air into the sparkplug hole.. You should see or hear air bubbles in the gearbox filler hole if a gasket is blown.
(20 PSI is a wild guess, but i think it's enough.. and it seems safe since a running engine develops way over 120 PSI in the case.. If someone can make a more educated guess, have at it.)
That broken gasket would be the one that splits the engine case down the middle, and that joint would cut right through, into the crankcase. If ATF is getting into the crankcase, it would get in by being sucked through a break in that gasket.