Mustang II suspension.
There are kits to cut out the spring.shock tower, and replace with a flat stiffener plate,and install a Mustang II type design suspension with better geometry than the Falcoln based 65 design. that will give you enough room, and you will not have to look for the 67 FE Mustang. exhaust manifolds. When they installed The Fe engines in the 67's, usually the exhaust manifolds put a big gouge down the firewall. In the factory it was a 3 man job, one man on top each side and one guy in the pit under the assembly line guiding the tail shaft. I would Make a sub frame kit for the 65, similar to what you can buy for the 79 and later Mustangs, Go for the ladder type with a tube under the rockers as well ad the ones under the seats front to back. . I made mine. The 65-66 mustangs with the K high performance engines used thicker guage Convertible part number rocker panels. K engine cars also had more welds, A Fe is heavier and more horse power will put more stress on the body shell. Convertibles also have a curved inner rocker panel reinforcement with a flat plate between the inner and outer rockers. a box section reinforcement under the front seats and a bolt in 1/4 in thick tie plate to tie them together. You can use cross pieces in that area from your sub frame inner rails to tie it together, I would bird cage the front structure with tubing, tubing box at the radiator support, along the top of the inner fender panels then use the Mont carlo type bracing fire wall to mid point of the side walls close to where the shock towers were, then the side to side top brace. ( ps I was an engineer at Ford during those years ) and you will also want to change to a 9 in rear end.