Oil at the rockers - if you have a Y block using a cross-drilled camshaft then you will need to rotate the cam to see oil at the rockers. Do you know which camshaft you have?
On the oil leak at the rear - it sounds like you have rear main seal leak. If you purchase a rebuilt engine, that should have been installed correctly. I would recommend you contact the company.
Valley Pan leak - did you install this yourself or did the company who sold you the rebuilt engine install the pan? over the years these valley pans get bent out of shape by mechanics who torque them down way too much. Unfortunately, you will need to remove the intake manifold, remove the pan, and reseal. I would suggest that you clean the mating surface of the block and the valley pan, and then use a straight edge at different points around the mating area to find where it's bent. You could use more sealer in that area if you could not straighten it out.
Lastly - where are you located? maybe someone on the forum not too far away can come over and help. We do it all the time.
Daniel JessupLancaster, California
aka "The Hot Rod Reverend" 
check out the 1955 Ford Fairlane build at www.hotrodreverend.com