Data plate mounting help

Hello everyone,
I also would like to know if anybody has any suggestions on tire size for my sillys. It has 31 X10.5 R15s on it now, and I am wondering if I should stay with that size or go to a somwhat smaller size. It has the orriginal F head engine in it. I do not do any harsh w wheeling. I drive a lot on pavement and also on BLM roads in AZ. Sometimes we get into some sandy conditions and some gravel, but that is about the extent of it. I would like the tires to be available for off road when I need it, but quiet enough so that driving on paved roads is not unpleasant. I tow the vehicle behind our motor home from time to time also. Thank you
Darryl