IMHO: swap it. Unless you like sewing machine noises and no balls under the hood.
Now with that being said - 273 is a bit costly to build power and you can only go so far with out bags of cash…. I know, I’ve got a built 273. If I had to do it again… I’d go built 318 for the money in my 66 Barracuda.
The v8 swap is actually really easy if you buy a parts car of the same generation and all the parts are there. Center link, throttle cable, manifolds. But if the 65’ has the 64-65 heads intake bolts can be an issue.
Remember - “imho” I stated that.
Back to you original poster.