I don't remember if either Haynes or Chilton books were available back then, but I had a 1970 Mercury Cougar Eliminator with the 351 Cleveland 4-bbl, "top-loader" 4-speed transmission and the 8-inch rear end with 3:51 gears that I bought in 1973. IIRC, I did have an 'aftermarket' repair manual, but as it was lacking on many subjects I worked on myself I splurged and bought the factory service manual for the car.
Boy, was that book an eye opener! It had information in it that unless you were a Ford/Mercury/Lincoln you would never know. If you had to drop the transmission to repair the gears, it had a listing of what regular and special tools needed to work on it, torque values for every bolt and nut, tolerance dimensions and clearances, tricks and shortcuts for disassembly and assembly and a fault symptoms-check/repair chart for each transmission used for that year and model. Every system of that vehicle, every option and every model (basic, XR-7, Eliminator, etc.) was covered in those books. Even FoMoCo supplied aftermarket parts and systems were covered: Edelbrock intake manifolds, Crane and Isky camshafts, Holley carburetors, Hooker headers, Cherry Bomb mufflers, etc. Remember, this was during the high performance wars between Ford, GM and Chrysler for street racers, NASCAR, NHRA, CAN-AM and other racing venues. These were the "Race on Sunday, drive on Monday" days of racing before insurance, safety and pollution concerns neutered high-performance cars.
If you are planning to "do it yourself" maintenance, repairs and upgrades to your vehicle, and keeping it for over five years, buying the FSM is a very good idea. It will save you money when you fix that one problem that takes you five minutes and a $10 part that would otherwise cost you over $100 for the dealer or repair shop to fix! $60-100 an hour labor costs and a $10 part costing $50+ through the dealer adds up in a hurry.
Regarding YouTube videos on car repairs: they are good for showing you visually how to do a repair/replacement, but have the correct book/s on hand so you know the correct way to work on the problem, have the right tools and the correct torque, tolerance and clearances values.