The easiest V8 to put in a 62-67 Chevy II would be any small block Chevy. They are physically the same size externally so anything from a 265-400 sbc is the same fitment and interchangeable.. The most common displacement available will be the 350 which started in production vehicles from 1969 through the mid 1990’s and continues to today in replacement crate engines..
I'd imagine the least amount of change would be an original '64 Nova option 283. But since that isn't going to happen, I'd get a simple (quality) rebuilt 350 locally or a Goodwrench 350 from Jeg's.
You'll need a different oil pan and oil pump.
A different arm for the accelerator pedal/carb linkage at the firewall.
Decide on either headers or manifolds, manifolds will be simpler and cost much much less.
Decide on points or HEI and rewire accordingly.
Same goes with the charging system.
The small blocks are the same physical size, mostly. Some early ('55) small blocks mount differently, and there are some other more minor changes. I used to think a 283 would be the cheapest option, but I don't think so anymore. 350s are so prevalent, so many were sold new and aftermarket that they're plentiful. An engine rebuilder recently told me that a complete rebuild kit for a 350 is cheaper than any other just because they can mass produce so many of the same thing that the cost is less, so it's a cheaper rebuild from that standpoint. If your car has a manual transmission, you might want to consider a Nova block (283, '64-'67) for the clutch linkage, but a 350 can be used with an adaptor the the clutch pivot. 350 with a 350 trans might just be the cheapest setup there is...that or 350/PG.