I wouldn't worry about it. That 273 is a virtually indestructible forged crank small block. How often do you see a factory engine that makes more horsepower than cubic inch size? Going purely by the description of the seller that 1966 273 is better than new mechanically and then you add a port matched LD4B to the 66 heads it should breathe well. Find a stock 5.2 or 5.9 engine that makes as much horsepower as that already complete 273. My wife and I have 9 cars with commando 273's. I wouldn't pull one of them for any other LA small block for what your end goals are!