Swapping a 273 for my 225?

It's essentially a bolt-in swap with a couple of exceptions. The 63 has a different wiper motor mount on the firewall that was revised for 64 to accommodate the V8 (distributor clearance). Also the 63 only had a 19" radiator where the 64 and later cars would accept a 22" radiator. Later slant cars still had a 19" radiator, but the mounting flange on one side was 3" wider. Beyond that it's the same as a 64-66. That is, you'll need the engine/transmission along with the front mounts, exhaust manifolds, steering center link, and gas pedal. The engine compartment wiring harness was also slightly different for the V8, especially the positive battery cable assembly.

The radiator support can be modofied fairly easily, as you can always make a hole larger. The firewall is a little more difficult. The most sanitary way would be to graft in that wiper motor mount area from a 64-66 car. Also, the V8 cars had cable throttle while slant cars used a rotating rod. Again, the 64-66 firewall had provisions for both, but a 63 could be drilled easily to accept the V8 throttle pedal bracket. If you have access to a 64-66 parts car, it should be fairly easy to just swap everything that's different.