65 Barracuda... Got BAD News...

Before you go through the trouble and expense of changing engines, there are a few things you need to determine:
1. Which cylinder, or how many, have either zero or very low compression?
2. What are the compression readings on the low cylinders?
3. What are the compression readings on the good cylinders?
4. Was the car running before all this happened, or has it been sitting a good
while?
5. Would you really, really like to put a 318 engine in it, and the low
compression readings give you a good excuse to do it? (I've used this
one!)

If you have a cylinder at "zero", it may be nothing more than a bad cylinder head gasket or a really burned/cracked valve. A piston with a hole in it will do it, but that doesn't happen a lot on low compression engines like the basic 273. A stuck ring is another possibility that careful analysis of each cylinder can determine as necessary. (does the engine smoke..a lot? )
If you've got a good, older model 318 laying around that you know is good to go as a bolt-in, go for it. If you have to rebuild either a 318 or the 273, the parts cost won't be much different...it's the machine work that will hit you hardest in the wallet, so which one needs the most machining?
If you want to keep it original, then fixing the 273 is the only option.
All these decisions...don't you just love old Mopars? Good luck on your motor.