Good points on both sides of the issue. The floorpans in this vehicle were quite rusty before I started working on it. Right now, the front sections have been completely rebuilt and reinforced with square tubing, and 1/8" thick sheet at the locations where the front down legs will attach. I am prepared to cut holes through the 1/8" steel if that's the direction I take. In the rear, there are rectangular holes, just slightly smaller than the 1/8" steel sections that will occupy those locations. 1/4" thick pads will go on top of the 1/8" sheet before the cage will be complete.
The body of the vehicle is currently mounted on a rotisserie. I plan to take it off the rotisserie and put it on jackstands for the cage fabricaition.
Originally, I was in the mindset of cutting holes in the floor to allow the cage to drop down/move around. I keep running this around in my mind, trying to figure out how to get ALL of the needed welds done by dropping the cage through the floor holes. Since this is my first cage build, I am worried about "welding myself into a corner", where I get to a point where I need to weld in another bar, but don't have access to the top side (like after welding in some floor or lower door bars). At that point, I'd probably have to cut the roof off anyway.
Someone here suggested that it might be easier to cut the roof off. I certainly agree that having full access to the top side of the cage would be much easier with the roof removed. However, it's also a good point about the safety of the factory-built A pillars, in particular. Making an inner sleeve for the A pillar, and rosette welding is a good idea. I could also weld some gussets between the cage and the A and B pillars to further reinforce the cut-apart and re-welded joint.
Anyway, I'm talking in circles right now, but appreciate the input on both sides of this issue.
Steve.