Oil Leak Coming From Transmission Tunnel

If you used the billet rear main from 440 source its entirely likely that it is the rear main leaking.
I had trouble w/ the main seal cap being milled off more than necessary when my machinist line honed the block. Was leaking between the gasket and cap. also had to machine the rear un-used part of the girdle off, right below the # 5 main cap, it doesn`t do anything but catch oil. hope this helps ??