Semi Slicks on 4,5" backspacing?

do you mean really "semi slicks" for round course or drag radials? My friend is one of the biggest dealers of these "semi slicks" (Toyo R888 for example), these are not the best tire for drag racing.

If you mean drag radials then it´s ok....

From what i measured at my rear axle 4,5" Bsp should fit, 4" Bsp should be ideal. I think your springs are in the same location, maybe the outer wheelhouse is bigger / wider compared to my valiant. My brother runs 8x15 / 4"Bsp on his Dart with 255/60-15 - it´s ok after trimming the outer lip of the wheelhouse. If i remember correctly i ran 255/60-15 on my old duster with 4"Bsp - no Problem.

Michael