Pressure test the crankcase first, to positively locate the source of the leak. I'd hate to see you spend 4 agonizing hours changing that seal, for it to change nothing.
I pressurize through the dipstick tube at 3 to 4 psi, after having blocked all escape routes; such as the pcv port and valve cover breathers. Then a soapy water test will find all leaks. Pay particular attention to the oil-pressure sender(or can if equipped), the dizzy base, and the back china-wall. Of course if the tranny is off, that makes checking the rear mainseal all the easier. Also check the intake manifold to head seal, the front china-wall and the fuelpump mount. Along with everything else that shows signs of seepage.
good luck