OK, here's the deal:
The allowable rim width range for an ST235/80R16 is 6" to 7 1/2".
The allowable rim width range for an LT235/85R16 is 6" to 7 1/2", but the tire is 1" larger in diameter - so you will need 1/2" more clearance.
The allowable rim width range for an LT245/75R16 is 6 1/2" to 8". It is 0.3" smaller in diameter, but it is 0.3" wider - so you need a little more than 1/8" side clearance, if you are using the same wheels.
Please note that tires move up and down due to suspension movement, but they also deflect sideways under cornering. The clearance you see when the vehicle is not moving is not what happens when the vehicle is moving. It takes some experience to determine how much you need.
Also, if you use a rim that is outside the allowable range, you MAY experience some issues. Common complaints are center wear, sensitivity to cross winds, and following the ruts and grooves in the road.
Personally, I think replacing ST235/80R16's with LT235/85R16's makes a whole lot more sense than LT245/75R16's.