weld prostar rear size
 

will a 15x8 with 3.5 backspacing and a 28x9 slick fit under the back of a fourth gen camaro?

not from personal experience but with a 15x8 idealy you want a 5.5 backspace... so with a 3.5 the tires are going to stick out about 2 inches further than they should... it would fit but may rub the 1/4 panels an will most likely look like garbage and a 28 inch tire will fit under a 4th gen no problem with stock rear springs.... but if you go with the 3.5 backspace it will probably rub due to sticking out to far

i believed i used 5.5" w/ 3/16" spacer for my 29''s

Simply, No.