289 oil leak. need information

It can be done with the engine in the car. You will have to remove the engine crossmember,(large bar running from side to side under the engine), and maybe a few other items. This is a fairly common problem. It sounds like the previous owner had the right idea but I think you also need to use a small amount of sealant at the sides. Hopefully someone that has done this will bring more detail.

could it not be the rear oil pan seal as most of the oil pan gaskets are actually 4 pieces???

Yes it can be done.
Get an inexpensive manual, nothing hard about it-but it will outline the precaution's and explain the way to do it, in better detail then I can.

The info on this link might help.

http://forums.stangnet.com/showthread.php?t=642563

You can replace the 4pc gasket with a newer Felpro 1pc. It's a LOT easier to install. Comes with 4 plastic studs. Scew the studs into the block, pop the gasket in place, lift the oilpan up on the studs, start the bolts. Tighten the bolts. You're done. Also helps to add a set of pan rail stiffeners from a late model 5.0 to help seal and keep the pan rails straight. If you do, get the late oilpan bolts to go with it, they're longer than the old ones. The plastic studs are made such a fashion where they hold the parts in place while you retreive the rest then bolt it up.

I played with what I thought was a rear main seal leak. When I removed the bellhousing and checked the back of the block, it was also dry except around the bottom of the crank. I carefully replaced the main seal and yet the leak continued. It appeared to drip only from where the pan meets the bellhousing.

What I later found was that the intake manifold had a small leak at the rear mating surface, right next to the head. The oil took the long way to get to the pan by following the bellhousing to block flange on both sides. No oil was going straight down the block.

Moral of the story is be sure that the rear of your intake manifold and valve covers have a good seal too as they may actually be the culprit when chasing a rear main seal leak.

Another thing, be sure that it is engine oil that is leaking and not tranny fluid.