I HAVE OIL LEAKING FROM MAYBE THE OIL PAN. IF IT IS NOT THE OIL PAN WHAT ELSE COULD IT BE? HOW DIFFICULT IS IT TO REPLACE THE OIL PAN AND WHAT EXACTLY DO I NEED.  

If it isn't the pan, it could be the rear main seal.  This would leak at the rear of the pan.  Basic hand tools (sockets, etc.) should get you through this.  You will need a pan gasket set.  If the rear main seal is leaking, new game...  

If it is the rear main seal is that a major repair?  

I read somewhere that the rear main seal can be replaced with the engine in the car by loosening the main bearing caps and working the upper half (The 66 289 has a two piece seal) around.  I have never done this and can't comment on the feasibility of doing so.  I have replaced one when I rebuilt a 289 and the engine was out of the car.  Hopefully, one of the others on this forum can advise you on replacing the rear seal with the engine in the car.  

It is possible to replace the whole seal, but its difficult. Its fairly easy to just replace the lower portion of the seal. In order to get to the upper portion you have to loosen/remove the rear main bearing cap and push out the old seal(you can tap it out with a punch and then grab it with pliers). The new seal can be worked around the upper portion of the block. There are various methods described to get this seal all around the upper portion but suffice it to say its difficult to do. Lots of patience can get the job done. The seal area must be completely clean or a leak will result where a piece of the seal or some sealer remained from the past seal. Rear mains are hard enough to seal even when the engine is apart and you can carefully clean/position the seal. I would say that most leaks are the rear main seal, maybe about 70% and the other 30% are the pan gasket,,, you pays your money and you takes your chances.