I've got a 351w in my 65 fastback. I'm using the original 289 mounts, but they are not K code mounts. You will need accessory mounts for a 351. Hood clearance will limit your choice of intake manifolds, but not too seriously. Custom hoods can also help with this issue. Your choices of headers are also limited, but there are still some good options.
I have way more questions for you at this point than answers. What kind of transmission do you have or are planning to run? What about the rear differential? Is it the original? Do you plan on upgrading the gears, axles, or upgrading to posi? What are your plans for the cars usage (cruising, street/strip. drag, autocross)? How much are you willing to modify and/or spend?