Just remember that whenever you cross one breed with another your get hybrid vigor. This is why crossbred calves weigh more at weaning (in general) than purebred calves. It affects everything from growth to longevity. As far as hybrid vigor, it doesn't start at birth, this can make heavier BW's too. Now having said that, you can still safely crossbreed heifers we do it all the time. Just pick the right sire for your heifer. Simmental isn't considered a calving ease breed, but I am certain that there are bulls out there that would be safe to use on an Angus heifer.
First off, what was HER BW? She's at least half the equation, after all her genetics are going to affect the calf and so is the enviroment she is living in for the 9 months she is CARRYING the calf. If she's a higher BW, you will want to consider going with a real proven calving ease sire. As far as environment, well, that is one thing you can't do much about. But have here in good BCS going into her last trimester. That is the worst time to try and put weight onto a heifer. And once shes into it feed her properly. She doesn't need a lot of grain, you more or less just want her to maintain herself and continue growing the calf. Things can go wrong no matter how good you try to plan them. But make the best plans you can and hope for the best!! If you do that everything will probably work out the way you want it to.