On my 68 the rearend is 56" Rotor Hat to Rotor Hat Outside. My Forgelines are 18x11 with 5.5 BS. It is tight to put a 18x12 with 335 on a 68. A few guys run an 18x11.5 with 335 just to be safe. You better measure a few times. Every car is different so you need to measure your car. Here is a cell pic of my car

Thanks, So my question is did you order your wheels first or your rear end?

I ordered my wheels/tires first. My rear end width is 57" WMS to WMS the wheels are 18x12 335s with 5.9" back space. Here are a couple of pics. Guy's rear end is 56" so hopefully you can start to see that each build is different which is why you need to make your own measurements on your car with your wheels.

http://hayes-ent.com/steve/images/Camaro/Camaro1.jpg

http://hayes-ent.com/steve/images/Camaro/CAMARO2.jpg

I love your car, so you ordered you wheels first what made you pick 5.9 for your backspacing. most people say 5.5 or 6. I also had someone suggest 6.25for rears. thanks for all the help

I ordered my wheels first. Mocked them up, hung string from the quarters and measured wheel to wheel about 10 times to make sure. Had my chevy rear narrowed and installed ford ends with bolt in axles. I will PM you my number in case you have any questions. Like JDMan said each car is different, I talked to him and a few others prior to narrowing my rearend. You have to decide how much lip you want on your rear wheels and the distance from the inside quarter lip and if you are trimmimg the lip. I am wierd (anal/paticular/OCD call it what you want) but I wanted a step lip portion of the wheel to be the same measurement as the flat portion of the wheel. That is how I came up with my backspace dimension. See attached image, the blue line and the red line are approximately the same dimension, about 2-1/8" each. I believe you can change the center spacing in 1/2" increments.

Guy