The oil pan will come off. I have been told (and it is in the shop manual) that the rear seal can be replaced with the engine in the car, however I have never done it.
Shop manual says it can be done. Basically it says to loosen the intermeadiate bearing cap screws (2 turns, more and you can damage the front bearing) and remove the rear cap. Rotate the old seal out, clean grove and rotate new one in around the crank. Retorqe bearing caps. Having done this with the engine out of the car and upside down, it sounds like a arm tiring job. Replacing the pan will be more "arm tiring" than replacing the seal.
I would suggest you wait to here from someone who has done it before, but it can be done. There are lots of experienced and helpful folks on this forum. Good luck
Just did one in a 58 Hawk last fall. Oil pan will clear tie rods. Remove the starter. Drain oil, remove pan bolts. Remove rear main. Push old seal out. oil new upper half and slide it in. Replace lower and install rear main. Torque to 90 F.LBS. Took me about 2 1/2 hours. Jim