want to put 17/8 wheels on the rear of my 92 caprice.what backspaceing do i need,and the largest width tire?

I can't answer myself, but here are links that may help you determnine what you'd like to know:

http://www.rsracing.com/tech-wheel.html#backspace

http://www.skulte.com/adapterfaq.html

http://www.getahelmet.com/jeeps/tech/backspace/