Advice Again

[314Chevelle]

I originally was going to do a ground up build on my 67 Chevelle so I started cutting the floor out & have them fully out all the way to the back seat. Later on I decided to do a frame off but I hadn't put the floors back in yet & I'm going to need a full trunk pan. I'm no pro & this is my 1st build so b4 I lift the body, should I put the floors back in or can i just lift the body? Will it flex or bend & should I tack weld braces ????