I did that on my SS, I moved the stock tubs inboard 3 1/4" and had to modify the seat back frame by narrowing it the same amount. Just cut a 3 1/4" section out of the wire on each side and moved it in and rewelded it. Like COPO said, I had to trim a little of the foam off. Didn't have to alter the cover, just took it off and put it back on after altering the frame. The way I figured it, 3 1/4" was about the max I could move the tubs in and stlll use the seat without major modification. In my case I moved the frame rails in 3 1/2" so moving the tubs in 3 1/4" made them even with the frame.