I don't understand why you would want to get rid of the original 12 bolt on a car with that kind of value. You can always put a big block back in it someday and have something worth alot of $$$. JMO
Don't sell the 12 bolt, later on in life I'm sure he will have the money to fix it up. I know someone who has a 69 model 396 that he wants to sell. If interested send me a pm and I will give you his number.
Save the 12 bolt and drop in a 9 inch Ford. 9 inch Fords are strong and still plentiful. If you sell the 12 bolt , you'll probably regret it someday. They are already hard to find and if he decides to sell it whole, he'd get a much better price with the original 12 bolt.