Hello Everyone!

I have been restoring my grandfather's 1947 CJ-2A, but I've run into a problem. The starter turns the engine over when the spark plugs are out, though slower than it should. When I put the plugs in, the added resistance prevents the starter from moving the engine. I hit the switch and the fan moves a couple of inches at best. When I ran into this, I sent the starter to Kaiser Willys to be overhauled. The problem persisted, so I sent it back. When they tested it, it was perfectly fine. I've tried using jumpers to go straight from the battery to the starter, but it made no difference.

The engine and transmission have both been overhauled (the power take off was removed when the transmission was overhauled).
The battery is new and charged.
I've replaced all the cables and made sure everything is well grounded.
The engine has oil and the cylinders are lubricated.
The transmission and transfer case are both in neutral.

Does anyone know what might be causing this? Could I have done something thing wrong when installing the transmission that would cause this friction, but would not stop the engine entirely?

I've run the starter for a about a minute straight to see if it just needed to move some before putting the plugs in, but there was no improvement. I'm worried about trying again if whatever is causing this will damage the engine.