Not sure if your overfenders are wider than 34s, if you had the option of going 0 offset, I would have built the guards around it a bit more.
+12s will fit flush/slight poke in the guards, but becareful, you may have rubbing when turning in the car - best bet would be to try them on if possiple.
Word of warning, 19" tyres, especially in 275/285 are real expensive.