I would wait until I had a frame before I would weld the floor supports in place. That way you can place the cab on the frame to ensure that the holes in the floor and hoes in the cab support matched the holes in the frame.
You can secure the floor supports to where you believe they should go with a few small self tapping screws for a test fit.
I wondered about doing it that way - just trying to not go through that unless I had too. However, it seemed that any of the measurements I have taken have been off a bit so there might not be an option to doing it that way.
