You can let it go above 70, you just probably won't like the result. I use 320, but keep it between 62-64 in a fermentation chamber (keezer). I'm not saying it won't be drinkable, just that you may get off flavors you aren't wanting.
Offhand I can't think of a wheat yeast that does well above 70, unless your specifically trying to get more banana/clove character. If you just want a clean beer, throw some US-05 or Nottingham in.
You may have to sanitize a long handled spoon and rouse the yeast (320) but be careful, and you may wake it up by getting it in suspension.