I would take the floor out and started with the cross member, then the rails, finishing with the floor. I have the floor out of mine, new cross member is in, just waiting to put the rail in to put the floor back in. It's alot of work, make sure to measure before you do anything to get a good baseline of how square the car is. Crib and support the car so it doesn't move on you, and get the factory measuremets from the service manual. Mymopar.com has the manual forr download. Best of luck with your project!