As noted above, do the jobs one at a time. Don't go hacking both floors out at once. Do one side, then the other from start to finish.
My project car had both floors replaced, the RH chassis rail, battery tray area on inner wing, and sections of bulkhead for RHD conversion. I did the bulk of that work with the car on axle stands. I then moved it to the rotisserie (I made arms for the front end that run along outside the chassis rails and bolt to the front crossmember mounting points, rather than attach the rotisserie to the front bumper points). Once on the rotisserie, I then finalized work such as installation of the under floor supports, tidying the floor welds, etc. All was done without bracing, other than fitting the front crossmember and the gearbox crossmember during the phase when the car was on stands. Thus, the car was not hanging in mid-air by its ends with substantial sections removed.
Worth considering the above as another approach?