Prestley Blake turned 100 years old last month, cofounded the Friendly’s restaurant chain with his brother 79 years ago, and probably figures he can do whatever he wants at this stage of his long life. But when he told his wife, Helen, what his swan song would be, her mouth dropped. “I want to build Monticello,” Blake declared. And so he has built it, $7.5 million later, in a meticulously crafted re-creation of Thomas Jefferson’s architectural masterpiece.