I know you said the starter is rebuilt but it sounds like a brush or commutator issue to me. I had the exact same struggle.Plugs out she spun like a champ. Put a load on the starter and the brushes were not delivering enough current. I did new larger battery, new heavier cables, cleaned up and confirmed solid contact on cables to battery, frame and starter. It was the brushes in the end. Oh, 12 volt system BTW...

Last edited by bztguy; 12/11/2013 11:23 PM.
1949 Chevy 3600
1975 W-25 Hurst Olds
1970 GMC 1/2 ton Fleetside
2010 Chevy Silverado