Move Rear Axle Back with Coronet Spring?

I'm about to relocate my springs inboard and wondered about moving the rear back at the same time to facilitate tire fitment. Found TheSuspensionKing.com on the net and notice the 66-72 Coronet spring center bolt is an inch more toward the back. The spring is also an inch longer but the rear shackle could be moved back too. It will be a street car so I don't much care about maintaining the stock wheel base to satisfy any sanctioning body. Seems like e-brake cable, hydraulic flex line and shocks would be fairly easy to address. What am I missing?