AllAroundCarGuy asked a question. I gave an answer. I never said it would be easy. Why does everyone overlook what Ford did in 65-66 for the K-codes? (and Shelby) They took the stock 8" rear and installed a 9" housing center into it with a 28 spline chunk. That keeps EVERYTHING the same, just upgrades to a 9" chunk. Costs about 200 bux and the donation of your 8" rear. I don't know about you folks, but I can find a 9" housing from something without much looking. Cut the center of the housing out, take it and your 8" to someone who can do the deed, and be done with it. A dirt track chassis shop here does it for a bit over 2 bills. Bolt in, stock length, keep your brakes...
Honestly though, why does one need a 9" anyway? Do you know the torture an 8" will take? If you're pushing over 500 hp, OK, go 9" Mild 289/302? Save your money for better upgrades. Besides, a 9" adds a lot of unsprung weight. As I mentiond earlier, or maybe somewhere else, I did the Versailles 9" disc install in my '66 GT Fastback. Wish I never went there, too many lower cost and better options.