Yep, most likely it is the rear main seal. The 216 CI six (and other early engines) is notorious for the drip, because of the early style "wick" seal. I was told by one gentleman who sold Chevrolets at a dealership back in the early 1950's that the Chevy dealership gave each buyer of cars and trucks a cardboard mat with a bow tie emblem on it to place under the vehicle when parked at home to protect the driveway/garage floor.
The book is correct, the lower half of the rear seal can be easily replaced by removing the main cap. The upper half is a bit of a problem and requires removal of the crankshaft to install a new upper half seal "by the book". Replacing only half of the seal usually does not fix the problem because the seal upper and lower halves wear at about the same rate. If it is leaking, the entire seal is compromised. There is however a product on the market known as a "sneaky pete" (google search this item) that can be purchased and used to replace the upper half without removal of the crankshaft. The degree of success is unknown, some swear by it, others do not. I have the same issue with my 51 truck engine but don't really get overlly concerned about an occassional drip, if they don't leak, they are out of oil!!
