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Bronco graveyard has them and they are about 15 bucks cheaper. I am using L&L for my conversion just because I know for sure they are the right thing and I'm getting a good product.
Mine were made by Hooker Header Corp. It was a long time ago so I don't even know if they are still available. I purchased them at Performance Automotive Warehouse. I also got the Hooker Competition Headers to match.
I really don't think they would be hard to make, if you had time and light metal manufacturing skills. Pretty simple design, but stout enough for sure.
I installed them in a 77 F150 that had a 351M From Performance Unlimited's site: "In order to position the main stud directly in the center of the mount for maximum strength, an additional hole drilled 2" forward on the tower is all that's required (sometimes simply moving the towers forward is even easier, or take advantage of the engine being moved rearward). No additional modifications necessary."
It would have been a lot easier had we not had the engine hanging in there already. Also, I believe going from a six to a 460 requires different towers.
On a side note, I had the L&L mounts in my 79 for less than 2 years and the rubber separated from the steel.