Ditto.Bud, I'm pretty sure your stock 16" steelies were only 7" wide... in that case that tire will not fit your rim. The largest tire I've found for a stock 16x7 steel wheel is the ProComp AT in 285/75/r16. That said, a 34X12.5 and no lift is going to rub offroad... and maybe even on road. Also the 6" of backspace on the factory wheel is not enough. You will need about 4" of backspace or wheel spacers or your front tires will hit when turning and rear when flexing. You be knocking off your flares man! ;O)
IMO, yes, it's a bad idea. If your set on a 34" tire you'll need new rims and a spacer lift, at minimum.