No...it can be done in the car. Need to drop the steering linkage, and remove the pan.

Thanks. I am getting some diffferent answers on the procedure. I just had the engine overhauled and it is leaking fresh oil from the general location of the rear seal. I would hate to have to pull the engine back out of the car.  I am going to continue to look into the problem. Hopefully between my mechanic and the overhaul shop they can find the leak. 

Sounds like whoever put your engine together needs to have it fixed.  Not you.

The engine  has a couple a hundred miles since overhaul. I have been driving it to get some time on the engine. Will the new rear main seal start to seal better after I drive it more or should there be no leaks?

It's not going to stop leaking if it's leaking now. The rear main seal is a rope type seal originally, but replacement seals are rubber. If not installed properly, either type will leak. To replace it, you'll have to remove the oil pan, the oil pump, the rear main cap, and loosen all of the main caps, to allow the crankshaft to drop down just a smidgeon. This will allow the upper seal to be replaced. I sure hope it has the rubber seals....the upper rope type seal is a major pita to replace...
The trick to making sure the seal doesn't leak is to off-set the ends of the seals, so that the seam between the upper and lower seals are not split at the same place as the main cap/block....one end should protrude from the engine side, and the other end of the seal should protrude from the main cap side. Don't know if that makes any sense without a visual aide....

Thanks for answering my question. I am going to have to keep after my mechanic and the engine shop that did the rebuild to make it right. 

Joel,  It sounds like the builder did not "Offset" the hard rubber seal (sold in the gasket set's these days) or not installed it with the seal "lip" forward to retain the oil into pan..

|UPDATED|6/27/2014 10:32:29 AM (AZT)|/UPDATED|

Update on the oil leak.    It has been a very long summer of dealing with different people about the oil leak. Very frustrating dealing with people that do not follow through with their work.   I finally got some answers to my questions .  The rear main seal is NOT leaking.  I have the car at a new mechanic and he told me after putting the car up on the lift that the oil pan is where the oil leak is.  I am thinking that when the engine was put back in the car that it was damanged.  I am going to have my new mechanic fix this after he finishes with installing the new carburetor.