I have never personally seen a JPJ calf be much more than the low to mid 70's...I have heard some bigger ones, but the heifers and cows throwing those bigger calves were giants themselves with pedigrees full of monsters. But like was said, they are all shaped right to come easy and I have never heard of a heifer having trouble with a JPJ calf, not sayin it hasnt happened, I just dont know of it. I think you can be about as confident breeding to him for heifers as you can for anything.
JPJ died yesterday though, so if you want some semen you better jump on it, limited supply left and sure it will go fast and get pricey. I am sure there will be a clone out soon though, so you can always wait for the clone.