ummm. quick question...are the original trunk drops (extensions)still
in place when you do this? or have they been removed?

If they have been removed,
then what do you use to support the A$$ end of the car?

If memory serves me right (and it's been several years since I last
did this job), on 68-70 B Bodies (non-charger) the down flange from the trunkfloor over the rear crossmember to the bottom/down flange of the tail panel is what supports the Quarter panels/rear structure..

If all of that is released (which is what you would "have" to do to replace the entire floor as 1 piece) then there is nothing in place to keep the rear quarters from "dropping down".

What are using to support/brace the rear to maintain your point of reference?

I don't see anything in the pictures...
Maybe I'm just getting old please correct me if I'm wrong....