They are the same width, height, depth, length etc... There is a minor difference in the gas tank straps and trunk support.
On 66-67 b-bodies they have one gas tank strap that goes side to side, SO there is a hole each frame rail to hold the strap.
On 68-70 b-bodies there are two gas tank straps that go front to back (from brackets on the trunk floor to the rear crossmember), also 68-70 b-bodies have a trunk floor support brace that goes side to side under the middle of the trunk floor, 66-67 cars dont have this trunk floor support. Hope this info helps.
