BEST is a relative term based on your expectations, or desires. That's wide open.

I believe the real question should be what engine would be the easiest to install since 61 has an earlier style frame.

Or what would be required to install an engine other than what comes in an early style frame?

Another good question is how much do you want to spend to swap engines. Or what do you want or expect from the engine you want to upgrade to and install?

If you get away from years that are a "bolt in" installation then you will have to adapt things like transmission, engine mounting & X member system, possibly the rear axle assembly and so forth. . .

For a bolt in you are limited to a Y Block style V8 [ 239-272-292 & 312 cid] and the 223 I-6 cyl or the 261 I-6.

All the other options, such as 289, 302, 351W or the FEs' or 385 series, and so forth will require modifications.

Another question is; How much do U want to spend on an engine swapping project ?

FBp