273 or 225?

If you swap it, this particular combination is going to present more issues than just the heads.
Assuming that they're both automatics, the '65 is going to be a cable-shift transmission, and your '66 is a mechanical linkage trans. Which means you'd have to switch to dash mounted push buttons or swap the steering column to the cable style column shifter (depending how the donor is equipped), swap out the driveshaft since the '65 uses a ball and trunnion output, the '65 uses a different leak-prone tough to source speedo cable, etc.
If it's a stick, you'd still have to switch the driveshaft, swap over the clutch linkage mount, and cut your floor and mount a hump (if necessary for a four speed).
On the plus side, you'd be getting the V8 specific throttle cable/linkage, the correct kickdown linkage, V8 motor mounts and exhaust manifolds, drop center link if your '66 doesn't already have it, V8 radiator, larger 10" brakes, and possibly the wiring harness. If the donor is a GT you could possibly get heavier suspension and a front anti-sway bar. As well as a ready source of odds and ends... If it is a very late '65, there's an outside chance it could have an 8.75 rear end in it, too.
Not to deter you from doing the swap if you decide to go that route, but just be aware it entails more than just dropping in a motor with the right mounts...