Is that the lower edge of the bellhousing, right behind the engine? And is the black thing at the top of the left photo the engine oil pan?
If so, it's nothing to worry about. There shouldn't be any fluids behind there at all, so nothing would be leaking through that hole either. The position shouldn't have any effect on structural integrity of the bellhousing, and I doubt anything would get up into there unless you are doing some deep water crossings.
If you DO get into deep water and mud often, then JB Weld would be fine here.
My oil line fix
My 100k stopped odometer fix
Brace for impact, I'm in a "modifying mood". Massive thread count inbound.