Swapping a 65 273 with a Later 318. What's gotta Change?

I think you'll have to use the 273 carb
If you have power-steering, make sure that you can see the timing marks on the 318 balancer. If you cannot; you have three options;
1) just reindex your TDC mark to somewhere else.
2) If the 273 has a removeable timing tab, just swap it onto the 318 cover and mark your balancer accordingly.
3) swap the entire front end off the 273 onto the 318. This allows you to use the original rad and pump.

I think the rest is plug and play. Well except for the 45 extra cubes, which will present as a lotta torque. So brush your teeth cuz you are gonna be doing a lotta smiling..... lol