273ci thoughts?
I just went through this decision myself for my 66 Barracuda. My 273 Commando is tired and needed to be rebuilt.
I decided to swap to a 5.9 (360) Magnum. It's not quite a straight forward swap as there are some differences. For example, to run a carburetor you have to swap intakes, the 5.9 is externally balanced so you need a new harmonic balancer to run the V-belts. Need a car oil pan, and I'm going with headers to avoid any clearance issues. An electric fuel pump helps too.
Here's some of the parts I've collected so far.