SB 727 troubleshoot

So, are you saying that you installed the tranny onto the engine? If so, did you correctly fit the TC; to the pump,and input,and stator support? And was the TC known to be a match to the tranny? The reason I ask is , it sounds to me like a problem in the pump. In which case it will have to come out.
Unless somehow that new filter is more or less completely restricting the pump pick-up.Guess thats worth a look-see.
As to the leak; that could be the o-ring at the bottom of the filler tube, coupled with the overfull tranny.