I am almost ready to attach my new rear half floor panel. With the car on a rotisserie, I can rotate to any position. My intuition tells me to position the car upside down, so that any flex will tend to "open" the door frames, and do it equally. With the car sideways, any flex will be asymmetrical. I do have door and diagonal braces welded in.

Any suggestions? Am I being paranoid?

Not sure, my rotisserie set up is different, but I agree to stay paranoid. Do your door braces allow you to mount the doors?

-Andrew

I'm president of the local chapter of Paranoid Chassis Welders. Do it with the car upside down. It can't hurt. I'm a big fan of taking any mechanical advantage available when welding on the chassis.

That said, welding in the floor isn't very high risk in terms of messing up the door gaps. Certainly not like welding in a stiffening kit or long repairs.

I think it would be pretty hard to mess up the door gaps welding in the floor. The important thing is to make sure all the dimensions are correct before any welding. My rotisserie didn't allow the car to go upside down so I welded the complete floor in on the vertical, but with solid longs, I had no measurable flex. That said, if I could have welded it with the car upside down, I would have. The bigger risk would be welding the floor in with the car wracked out of square but if you are only doing the rear section, there isn't much chance of that happening either.

I agree.
I welded my floorpan on with the car upside down.
OP your car is more braced than mine was, and I did not see any change in the floorpan gap before I welded, whether the car was sideways or upside down

If you are using a overlap joint make sure the joint is proper. Front half should be closest to you and the back pan should be closet to the road. That way if your in an accident and the pan halves seam fails the occupants in the car stay protected. Do not need to add a sheet metal guillotine to your project if you can avoid it.

I think Jeff Hail mentioned this a few times, stuck in my head for some reason.

If your door gaps are goodIt does not matter at all which way you good. Spread your heat out, and take your time you will be fine.

Thanks, everyone. Glad there are some others the paranoid club.

Here's a view of my braces. I mounted the front connection to the hinge bolt holes, so I can't mount the doors. I do have reference measurement points marked and so far I see no change. My longs are very solid on both sides, as are the rear door columns so I feel slightly less paranoid.

The seem across the car between the remaining floor and the new panel will be a butt weld.