Thanks Eric but that wouldn't help the existing hole which could leak. Perhaps that B&M kit is for more modern tranny pans that don't have any drain plug. They want you to drill a 1/2 inch hole. I already have a 5/8 hole. We successfully removed the bung in the rusty pan. It was only spot-welded two places on the top side of the bung. When I drop the good pan, we will remove that bung and weld in the good one. I'm going to look at my spare tranny - bet it has the same bung.

Keith, As I said in my original post I am putting the bung from the rusty parts oil pan into my good pan which has a stripped bung. Good hint at checking bungs to make sure they are not split.

Brian, the bung originally was just spotwelded in place. After examining the spare pan, it seems the sealing is done by tightening the copper or nylon washer snugly against the pan.
The hole in the pan just accomodates the 5/8-18 drain plug.
The plug that was in the 400 was an oversize one. I just want to get back to original. I will test it with water before I bolt it back up.
I wonder how long Packard initially allowed to install a new oil pan gasket? I don't have that manual although Kev may have posted it here.
A separate thought - I believe there is a bung in the transmission pan also. The drain plug there is also a 5/8-18.

As some of you know, I had an oil pan drain plug problem when I changed oil in November to give the 400 a long winter's nap. It turns out that the pan has had an oversize drain plug in it since I bought the car. I don't know what finally killed it but I only change oil once a year and the same mechanic always drains the oil and changes the filter. I dove under the tarp to recover the pan off the parts car but it is too rusty to trust. However the drain plug, which seems much smaller than the current one, had really good threads and was obviously smaller in length. Packard spotwelded a thick bung inside the oil pan and the body man at ther garage where I have oil changed says he can drill out the spotwelds on the bungs of both pans and install the good bung by rewelding it in the good pan. He said it would be about $30.00 which is much cheaper than installing a helicoil which was another alternative. The helicoil kit was $50.00 and I would have to purchase the required 41/64th drill bit separately. Dropping the pan is a PITA but I will be much happier with that fix. Sorry to be long-winded, but I wanted to explain the problem and the options in detail.