Got your work cut out for you. I happened to find a brand new one about 3 miles from the house on Craigslist a few years ago when I needed one. I was away from home for a month but it was still there when I got back for 20 bucks. It was a take off from a Ford crate engine that was going into a 60's Mustang. Here is the part number that may help your search D5UE-6675-BA. Originally for a mid 70's Econoline. It also might be found using OMC's part number 0982520. Don't understand why no one like Dorman has them. Good Luck in your search.
Another possible source for the 351 oil pan is Mercury Marine. They marinized and used the 351 for about 20 years in both inboard and stern drive applications. Some were made out of steel and others were cast aluminum. If you go this route make sure you get the oil pick tube up too. Best luck will probably be on the salvage market where someone is parting out an old engine or NOS (new old stock) that an old Mercury dealer still has on his shelf.