changing backspace

[seekins]

I found some Shelby speedline rims, but they aren't the correct backspace for my axle width. I would like to use them allong with the forgelines i like so i have 2 sets of tires for track day and daily driving. They are a 3 piece wheel but im not sure how much it would be or if its possible to change them?

I cant remember the backspace on the forgelines i want, would have to ask frank but it is his standard offset he recomends with 335's and has about a 5" lip to them..

Thanks,,