Did you make up new battery cables like you mentioned? I ran into a similar problem once and it was crappy and to small of cables. Even with a rebuilt starter it acted like you've described even with 2 sets of jumper cables from another fresh battery until I supplied it with fresh battery cables (Thats when I figured out there's like about 20 strands of wire in jumper cables and they carry 12 volts fine but they don't carry 600 amps for s^%t)
But that still doesn't answer why it's hard to turn by hand... I think I have to agree with Roy Rodgers and would have that pan off to know what IS going on from the bottom up at this point.
Not just looking at the oil but looking in the bottom of the pan and checking out the mains as others have suggested.
