65 Valiant swap 273 to a 360

The inlet to water pump on the 360 is on the opposite side. If you wish to keep the 273 rad then you will need to swap the entire 273 front end onto the 360; EXCEPT the ballancer which is 360 specific. This means you will need to reindex the ballancer for #1 TDC. This is not a big deal.
But IMO moving the lower rad spigot is a better idea. Then you get to keep the better late-style water pump. If the power steering pump gets to be in the way, then you have two choices: get the late style pump mounts, or back to swapping the frontends. When you start swapping these parts around, you may find that various pulleys don't line up anymore. This is a PITA but it just means getting the matching right stuff.
Personally, I would, and did, change over to the 70up style front end. And just got a matching rad.

The funny thing is, I did this type of swap in about 1975. I put a 69 340 into a 65 V100Wagon(originally a slanty) and used a 65 Valiant 273 tranny. And I don't recall ever installing the special spacer. Then again, I was just 22, so some would say I was stupid, or ignorant. And I probably was,lol. I guess I drove that car about 2 or 3 years that way, before the thing gave up. It sure was fun while it lasted tho!
With an extra 20 cubes and less weight, I bet your 360 will be a blast!!