I would never breed a Simmental to a Hereford cow. breeding a Simmental to a Hereford heifer is IMHO irresponsable.
Check it out yourself. look at what weight Hereford calves are and what weight Simmental calves are.
About 30 years ago, a neighbor wanted to increase the size of his feeder calves. He used a Simmental bull. He had the Vet out many times, preforming C-sections to save the calf. He also lost several calves and a few cows.
These are my experiences. I hate calving problems. The advantage of a larger calf is offset by the chances of losing the calf, Vet costs to assist delivery, C-section surgery, and the possible distruction of the cow's reproductive tract or outright loss of the cow.
Note that while the birth weight difference is only about 10 pounds, calving difficulties for Simmental/Simmental is five times higher than either Angus or Hereford calving difficulties. Clearly, if Simmental/Simmental have a significantly higher calving problem, using a Simmental on a Hereford cow will surely increase calving dificulties above that rate. Breeding a Simmental to a Hereford heifer, pushes the odds of trouble even higher.
Give it a try. You might get lucky. But IMHO, you'd have better odds playing Russian Roulette.
A shot of Selinium a few weeks before birth increases the strength of the calf and might allow survival in the event of a prolonged difficult birth.