273 -318 swap information needed

Since you don't yet have an engine, a 360 drops right in there too. With the only additional hiccup being the one engine bracket, which is easy to solve.
Well, also that the 360 is externally balanced. An aftermarket 360 external-balance flexplate would allow you to run a neutral-balance torque converter, but you would have to run a 360 damper up front. Couldn't use the one from the 273. Would have to run a 360 oil pan, too, but that should fit a 65 Barracuda OK.

A 340 would drop right in, but they are hard to find these days (I'm old enough to remember when they were easy to find in junkyards, but that was a long time ago.)