100 Ah is the battery capacity (think of energy storage ability), not the cold cranking Amps the battery is able to provide. Still, a 12V 100Ah battery should be able to provide sufficient CCA to turn a 413 engine, even more so if the starter motor has been rebuilt.
Too much oil in the cylinders can hydro-lock the engine.
When you installed the starter did you clean the mounting surfaces to clean metal on the starter and bell housing and clean the mounting bolts?
Did you run a jumper cable from the negative post on the battery to a starter mounting bolt?
Oil is not the problem. As said, when it's released from sparks plugs it runs fantastic, and any oil excess was released. Answering your second question, I even gave ground straight to the starter body from the battery, and same with the positive. But same.
Everything points to the starter motor. But was rebuilt by an expert, who checked it. And sadly dont have access to a second working motor to check that. Will see...
Check the cables and connectors. Those -50 starters are way underpowered when compared to later ones. My parents -59 dodge had similar issues which were mostly fixed with thick new cables and connectors (starter and block ground wire). Use atleast 50mm^2 cable for the starter.
Also are you completely sure that the starter itself is ok? All bearings ok? Not shorted field wires etc?
Does it still crank ok with the spark plugs in, but with the ignition disconnected? If so, try retarding the timing. But I've read those old direct drive starters are troublesome. Good advice given above to oversize the cables.
