The tubes above and below the crank hole are totally blocked by the hole. My radiator guy said it would reduce cooling capacity by about 10%. As Geo says, it can be made to work with the hole, But I opted to leave it out for the extra cooling. Modern cores are visibly different than the original anyway (the fins run zig-zag instead of horizontally) so anyone that cares is going to know it isn't the original radiator. And since I've never owned a crank that worked, I've never missed it.
Regardless of which you choose, I would suggest taking it to a radiator shop and insisting that they "rod it out". On my latest go-round with overheating, the radiator shop twice pronounced the radiator "good" without rodding it, and the engine would overheat no matter what I did. When I finally insisted that something had to be wrong with the radiator; they found that although none of the tubes were blocked, they were all coated inside with some sort of "mud" that was interfering with heat transfer. Very likely the remains of the stop-leak that I used to run because the extension always cracked where it joined the upper tank. But by the time they finished forcing the rods through, it leaked in half a dozen places, so I opted for a new core. The difference was like night and day; all my overheating issues just vanished!
You might also want to consider having them add some reinforcement around the extension joint. As I understand it, they bent a piece of small copper tubing to fit into the corner and then soldered it to both the tank and the extension. With a coat of paint it is practically unnoticeable but so far at least it seems very effective at preventing cracks at that joint. Others don't seem to have as much trouble, so maybe it's just the way I drive (or the distance, I still commute to work almost every day in my TR3), but I've had cracks at that joint many times over the past 40 years.