273 swap

For starters: V8 radiator; under hood ignition and starter wiring changes; different engine mounts; larger exhaust, possibly need larger fuel line from tank; carburetor linkage; and different bell housing as /6 and V8 have different bolt pattern.

Engine /6 K member to V8 engine mounts here:
http://www.engine-swaps.com/Pages/ProductsYear/63.66aBody.html

Also you may want to consider up grading brakes, torsion bars, and depending on how built that 273 is a heaver rear end. Just dumping extra power in and not upgrading the suspension, drive train, and brakes is not a good practice.