Not sure

Son, personally, I would stay with the slant six, if you are only considering a swap to a 273 or 318. That said, to your question about changing the "K" frame. It depends on the year of the car. 64-66 use the same "K" frame for the V-8 or slant six, so on those years, no change required. 67 and newer, requires either a "K" frame change or adapter motor mounts (http://www.engine-swaps.com/). 64-65 and some early 66's require a V-8 drag link. You will also have to use the matching V-8 trans, and radiator.