I am trying to fit a set of 15x8 wheels with Nitto 555s on my 89 Firebird Formula 350, wheels are jeg stars I think they are 5.5" back spacing... measures 5.5" from the back of the hub to the edge of the rim, I am not 100% sure on the wheel dimensions. They are hitting the inner fender hard core even with the bump stops cut out. Rear end is a Quick performance 3rd gen 9" with yukon center and moser 35 spline axles and 4th gen camaro ss rear brakes.

They appear to stick in quite a ways just mocking it up, I wonder If I should have gotten 4.5" back spacing? Or should I run a set of 1" spacers?

Any advice much appreciated!

 

"If" your going to drag I do not think spacers are recommended so I would look for a wheel that will work for you.
If there 5.5 it looks like (from the photos) you could pull off 4.5 as long as that,s enough.

Sounds like you measured correctly but just in case.
From the backside of the wheel =

 

Almost looks like a narrowed rear. Probably need less backspacing. 4.5 seems right for stock wheel width and rear width

 

I ordered a set of 1" hub centric spacers, we will see how they hold up

 

I think it will hold up. I have put 1000 hp to 5/8 hubcentric spacers