What Dan said. IIRC, the plug has an extension on the inside (longer than a standard pipe plug) that keeps the rear bearing in position. The service manual calls for adding 1/2 pint of transmission lube thru the plug opening whenever the u-joint bell is disassembled.
Your leak is probably pretty normal given a 70 year old bell gasket setup. When the u-joint fails (and it probably has over the life of the truck), the ball where the cork seal runs gets lumps on the outside, along with the normal wear of the movement of the torque tube as the rear springs flex. Also, you may not have installed the correct number of gaskets between the transmission and bell cover. There's a procedure to determine how many gaskets to use. But it will still probably leak.
Or someone may have plugged the vent on the rear axle, causing pressure to build up in there. That happened on my truck and the rear axle bearing seals were basically blown out.