I also have tried the one piece gasket w/o luck. The first one"fit" so poorly at the rear main I didn't even try to install it. The second fit better and didn't leak for a while.
I met and talked with a fellow Mustanger who had my exact issues. I told him I thought my leak was from the rear main. He too originally blamed the rear main, but said he put a dab of rtv at the front and rear corners where the pan goes over the mains. This trick solved his oil leak.
I haven't touched my car in 2 weeks, but plan on trying the rtv as the next fix.