You can easily rule out some of the causes for a leak in the rear of the motor. First, if the rear cork seal of the intake is leaking there will be evidence of
oil on the upper rear surface of the block directly below the intake manifold, easy to feel with your finger. If the
oil pan gasket is the source, usually there will be
oil present on the rear of the oil pan in front of the block plate. A flashlight will help here. If the leak looks like it is coming from inside the bell housing or mainly on the block plate, then it could be the main seal or the flywheel bolts but many times if the flywheel bolts are the source you will have oil on the surface of the flywheel and clutch problems as a result.
Early blocks use a two-piece seal while late blocks use a one-piece seal. Usually it is the two-piece seal that gives the problems. When replacing the two-piece seal, be sure NOT to align the ends of the seal with the joint made by the end cap and the block. The ends of the seal should be offset. To replace the two-piece seal you must remove the tranny to get the input shaft out of the way and then drop the oil pan. Then the main caps need to be loosened to allow the crank to drop down a little. There is a tool available to make removing and inserting the new upper half of the seal easier. The job is easier than it sounds but it can be time consuming.
I agree with everyone else who posted. Spend the time and effort to carefully locate the exact source of the leak before tearing anything down. Good Luck.