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.)
Also the 360 uses a different oil pan. It has the big main seal front and rear. I have a 360 in my 63 Valiant where a /6 used to be. I may need a different center link for to clear the oil pan.
Quite a bit more than just an engine mount bracket to put a 360 in a '65 A-body. Already mentioned is the external balance issue and oil pan (some pans need to be messaged a bit with a hammer). depending on the balancer, some pulleys don't like the external weight. Exhaust system is a whole nother bag of worms.
