The Dynacorn boxes are good in the one piece boxes ,others not so good . The two piece may be a bit easier to install . You will need to make a slice in the toe board on each side of the car about 1/4 inch by about 10 inches long for the torque box flange to pass inside the car .You will want to make sure the car is as level as possible so there will be no twist in the frame to rockers . It really is not hard to install them in a 65 -66 and i would recommend them for any 65-66 to help stop front end sag and twist and to add strength.
