Hi Mike,
We're assuming a 216 or 235. And if that is the case, we can go ahead and assume it is the rear main...but before you condemn it, please stick your fingers behind the engine head and wipe across toward you. See oil? Your valve cover gasket could be leaking and that will mimic a rear main leak. Also look down at the lower rear corner of the valve lifter panel. Is there oil there? It can leak right there and run down to mimic the rear main, also. You could have a leak at the oil galley plug, but that is a much different problem so we'll leave it alone for now. Actually it is somewhat less likely than the other culprits because it doesn't rust.
Which year, however? 1956 and later could have used the neoprene (well really they aren't neoprene but a different compound). We can call them that, though. If you have a 1955 or older engine, you'll need to use the rope seal and it might be worth your effort to not replace it...at least until you try the windshield wiper stainless piece trick.
Here's how it goes: take a piece of the stainless stiffening metal piece used in many windshield wipers and using the lower rear main cap to measure, remove the rope seal from it and cut a piece of that stainless the same arc as the bottom of the groove. Now file or grind a bevel on one end and just slightly radius the corners. Now using a small hammer lie on your back and tap that stainless piece in at the very bottom of the old seal piece, forcing it outward toward your crankshaft. Place it so the bevel you made faces the seal (so it will slide through without grabbing it. When you get this all the way through, then replace the rope seal in the lower rear main cap as the shop manual says to and replace the gasket & oil pan. Try it and see.
If you have the newer engine the process is a bit different. You'll need to use a piece of wood like a chopstick or a brass drift and carefully tap the center of the neoprene seal to force it to rotate around the crank and out. Usually I will use a box cutter or something to help it once I get about 1/2 inch out...you can try dropping the crank a bit, but if your transmission is in place you won't get much relief. When you replace that top neoprene seal apply a thin coat of Permatex 300 or similar anaerobic sealer. Be certain to point it the right direction...sloping side facing away from the crankcase.
