Just my opinion, I'd go with a 460. These are very common and parts are plentiful. These should bolt in. Second, I'd find the four speed of your choice and keep your clutch pedal. If you really want an automatic, get an automatic pedal assembly and sell the pedals to someone who can use them. They are not that easy to find.

As these are heavy cars and not road racers, keep the gear box and don't worry about the rack.