Hello. The sheet metal of the car is all the same. There are some little differences, like the hood latch bucket assembly and the grill, but that isn't a big deal. If what you are planning on doing is removing all of the sheet metal from the 66 and using it on the 65, the only thing that will complicate things for you will be the doors. The 66s door hinges use six 5/16 bolts to hold the doors onto the cars and the 65s use six 3/8 bolts. The clearance holes in the 65 door hinges are a bit larger, also. If you use the 66 doors on the 65 car, you'll need to use the 65 hinges so that they look like 65 model doors, except for the way that the door tag will be in the wrong spot. That isn't something that a lot of people notice, though.
There is also the fact that the simulated side scoops on the 66's have three prongs and the 65's have half moon shaped scoops. There is no 170 cid straight six in a 66.